The Vertebrate Skeleton

In the following pages the term skeleton is used in its widest sense, so as to include exoskeletal or tegumentary structures, as well as endoskeletal structures. It was thought advisable to include some account of the skeleton of the lowest Chordata—animals which are not strictly vertebrates, but it seemed undesirable to alter the title of the book in consequence.

The plan adopted in the treatment of each group has been to give first an account of the general skeletal characters of the group in question and of its several subdivisions; secondly to describe in detail the skeleton of one or more selected types; and thirdly to treat the skeleton as developed in the group organ by organ.

A beginner is advised to commence, not with the introductory chapter, but with the skeleton of the Dogfish, then to pass to the skeletons of the Newt and Frog, and then to that of the Dog. After that he might pass to the introductory chapter and work straight through the book. I have endeavoured to make the account of each type skeleton complete in itself; this has necessitated a certain amount of repetition,—a fault that I have found it equally difficult to avoid in other parts of the book.

Throughout the book generic names are printed in italics; and italics are used in the accounts of the type skeletons for the names of membrane bones. Clarendon type is used to emphasise certain words. In the classificatory table the names of extinct genera only, are printed in italics.

In a book in which an attempt is made to cover to some extent such a vast field, it would be vain to hope to have avoided many errors both of omission and commission, and I owe it to the kindness of several friends that the errors are not much more numerous. I cannot however too emphatically say that for those which remain I alone am responsible. Messrs C.W. Andrews, E. Fawcett, S.F. Harmer, J. Graham Kerr, and B. Rogers have all been kind enough to help me by reading proofs or manuscript, while the assistance that I have received from Dr Gadow during the earlier stages and from Prof. Lloyd Morgan and Mr Shipley throughout the whole progress of the work has been very great. To all these gentlemen my best thanks are tendered.

All the figures except 1, 35, 55, and 84 were drawn by Mr Edwin Wilson, to whose care and skill I am much indebted. The majority are from photographs taken by my sister Miss K.M. Reynolds or by myself in the British Museum and in the Cambridge University Museum of Zoology, and I take this opportunity of thanking Sir W.H. Flower and Mr S.F. Harmer for the facilities they have afforded and for permission to figure many objects in the museums respectively under their charge. I have also to thank (1) Prof. von Zittel for permission to reproduce figs. 27, 41, 52, 69, 70, 80, 106 A, and 107 C; (2) Sir W.H. Flower and Messrs A. and C. Black for figs. 1 and 84; (3) Prof. O.C. Marsh and Dr H. Woodward for fig. 35; (4) Dr C.H. Hurst and Messrs Smith, Elder, and Co. for fig. 55.

A few references are given, but no attempt has been made to give anything like a complete list. The abbreviations of the titles of periodicals are those used in the Zoological Record.

I have always referred freely to the textbooks treating of the subjects dealt with, and in particular I should like to mention that the section devoted to the skeleton of mammals is, as it could hardly fail to be, to a considerable extent based on Sir W.H. Flower's Osteology of the Mammalia.

SIDNEY H. REYNOLDS.

March 10, 1897.

By the term skeleton is meant the hard structures whose function is to support or to protect the softer tissues of the animal body.

The skeleton is divisible into

A. The Exoskeleton, which is external;

B. The Endoskeleton, which is as a rule internal; though in some cases, e.g. the antlers of deer, endoskeletal structures become, as development proceeds, external.

In Invertebrates the hard, supporting structures of the body are mainly exoskeletal, in Vertebrates they are mainly endoskeletal; but the endoskeleton includes, especially in the skull, a number of elements, the dermal or membrane bones, which are shown by development to have been originally of external origin. These membrane bones are so intimately related to the true endoskeleton that they will be described with it. The simplest and lowest types of both vertebrate and invertebrate animals have unsegmented skeletons; with the need for flexibility however segmentation arose both in the case of the invertebrate exoskeleton and the vertebrate endoskeleton. The exoskeleton in vertebrates is phylogenetically older than the endoskeleton, as is indicated by both palaeontology and embryology. Palaeontological evidence is afforded by the fact that all the lower groups of vertebrates—Fish, Amphibia, and Reptiles—had in former geological periods a greater proportion of species protected by well-developed dermal armour than is the case at present. Embryological evidence tends the same way, inasmuch as dermal ossifications appear much earlier in the developing animal than do the ossifications in the endoskeleton.

Skeletal structures may be derived from each of the three germinal layers. Thus hairs and feathers are epiblastic in origin, bones are mesoblastic, and the notochord is hypoblastic.

The different types of skeletal structures may now be considered and classified more fully.

A. Exoskeletal structures.

I. Epiblastic (epidermal).

Exoskeletal structures of epiblastic origin may be developed on both the inner and outer surfaces of the Malpighian layer of the epidermis[1] . Those developed on the outer surface include hairs, feathers, scales, nails, beaks and tortoiseshell; and are specially found in vertebrates higher than fishes. Those developed on the inner surface of the Malpighian layer include only the enamel of teeth and some kinds of scales. With the exception of feathers, which are partly formed from the horny layer, all these parts are mainly derived from the Malpighian layer of the epidermis.

Hairs are slender, elongated structures which arise by the proliferation of cells from the Malpighian layer of the epidermis. These cells in the case of each hair form a short papilla, which sinks inwards and becomes imbedded at the bottom of a follicle in the dermis. Each hair is normally composed of an inner cellular pithy portion containing much air, and an outer denser cortical portion of a horny nature. Sometimes, as in Deer, the hair is mainly formed of the pithy portion, and is then easily broken. Sometimes the horny part predominates, as in the bristles of Pigs. A highly vascular dermal papilla projects into the base of the hair.

Feathers, like hairs, arise from epidermal papillae which become imbedded in pits in the dermis. But the feather germ differs from the hair germ, in the fact that it first grows out like a cone on the surface of the epidermis, and that the horny as well as the Malpighian layer takes part in its formation.

Nails, claws, hoofs, and the horns of Oxen are also epidermal, as are such structures as the scales of reptiles, of birds' feet, and of Manis among mammals, the rattle of the rattlesnake, the nasal horns of Rhinoceros, and the baleen of whales. All these structures will be described later.

Nails arise in the interior of the epidermis by the thickening and cornification of the stratum lucidum. The outer border of the nail soon becomes free, and growth takes place by additions to the inner surface and attached end.

When a nail tapers to a sharp point it is called a claw. In many cases the nails more or less surround the ends of the digits by which they are borne.

Horny beaks of epidermal origin occur casing the jaw-bones in several widely distinct groups of animals. Thus among reptiles they are found in Chelonia (tortoises and turtles) as well as in some extinct forms; they occur in all living birds, in Ornithorhynchus among mammals, and in the larvae of many Amphibia.

In a few animals, such as Lampreys and Ornithorhynchus, the jaws bear horny tooth-like structures of epidermal origin.

The enamel of teeth and of placoid scales is also epiblastic in origin[2] , and it may be well at this point to give some account of the structure of teeth, though they are partly mesoblastic in origin. The simplest teeth are those met with in sharks and dogfish, where they are merely the slightly modified scales developed in the integument of the mouth. They pass by quite insensible gradations into normal placoid scales, such as cover the general surface of the body. A placoid scale[3] is developed on a papilla of the dermis which projects outwards and backwards, and is covered by the columnar Malpighian layer of the epidermis. The outer layer of the dermal papilla then gradually becomes converted into dentine and bone, while enamel is developed on the inner side of the Malpighian layer, forming a cap to the scale. The Malpighian and horny layers of the epidermis get rubbed off the enamel cap, so that it comes to project freely on the surface of the body.

As regards their attachment teeth may be (1) attached to the fibrous integument of the mouth, or (2) fixed to the jaws or other bones of the mouth, or (3) planted in grooves, or (4) in definite sockets in the jaw-bones (see p. 107).

Teeth in general consist of three tissues, enamel, dentine and cement, enclosing a central pulp-cavity containing blood-vessels and nerves. Enamel is, however, often absent, as in all living Edentates.

Enamel generally forms the outermost layer of the crown or visible part of the tooth; it is the hardest tissue occurring in the animal body and consists of prismatic fibres arranged at right angles to the surface of the tooth. It is characterised by its bluish-white translucent appearance.

II. Mesoblastic (mesodermal).

Dentine or ivory generally forms the main mass of a tooth. It is a hard, white substance allied to bone. When examined microscopically dentine is seen to be traversed by great numbers of nearly parallel branching tubules which radiate outwards from the pulp-cavity. In fishes as a rule, and sometimes in other animals, a variety of dentine containing blood-vessels occurs, this is called vasodentine.

Cement or crusta petrosa forms the outermost layer of the root of the tooth. In composition and structure it is practically identical with bone. In the more complicated mammalian teeth, besides enveloping the root, it fills up the spaces between the folds of the enamel.

The hard parts of a tooth commonly enclose a central pulp-cavity into which projects the pulp, a papilla of the dermis including blood-vessels and nerves. As long as growth continues the outer layers of this pulp become successively calcified and added to the substance of the dentine. In young growing teeth the pulp-cavity remains widely open, but in mammals the general rule is that as a tooth gets older and the crown becomes fully formed, the remainder of the pulp becomes converted into one or more tapering roots which are imbedded in the alveolar cavities of the jaws. The opening of the pulp-cavity is then reduced to a minute perforation at the base of each root. A tooth of this kind is called a rooted tooth.

But it is not only in young teeth that the pulp-cavity sometimes remains widely open; for some teeth, such as the tusks of Elephants and the incisor teeth of Rodents, form no roots and continue to grow throughout the animal's life. Such teeth are said to be rootless or to have persistent pulps.

An intermediate condition is seen in some teeth, such as the grinding teeth of Horses. These teeth grow for a very long time, their crowns wearing away as fast as their bases are produced; finally however definite roots are formed and growth ceases.

Fig. 1. Diagrammatic sections of various forms of teeth
(from Flower).

I. Incisor or tusk of elephant, with pulp-cavity persistently open at base. II. Human incisor during development with root imperfectly formed, and pulp-cavity widely open at base. III. Completely formed human incisor, with pulp-cavity contracted to a small aperture at the end of the root. IV. Human molar with broad crown and two roots. V. Molar of Ox, with the enamel covering the crown, deeply folded and the depressions filled with cement. The surface is worn by use, otherwise the enamel coating would be continuous at the top of the ridges. In all the figures the enamel is black, the pulp white, the dentine represented by horizontal lines, and the cement by dots.

The teeth of any animal may be homodont, that is, all having the same general character, or heterodont, that is, having different forms adapted to different functions. The dentition is heterodont in a few reptiles and the majority of mammals.

Succession of teeth. In most fishes, and many amphibians and reptiles the teeth can be renewed indefinitely. In sharks, for example, numerous rows of reserve teeth are to be seen folded back behind those in use (see fig. 15). The majority of mammals have only two sets of teeth, and are said to be diphyodont; some have only a single series (monophyodont).

Development of teeth. A brief sketch of the method in which development of teeth takes place in the higher vertebrates may here be given. Along the surface of the jaws a thickening of the epiblastic epithelium takes place, giving rise to a ridge, which sinks inwards into the tissue of the jaw, and it is known as the primary enamel organ. At the points where teeth are to be developed special ingrowths of this primary enamel organ take place, and into each there projects a vascular dental papilla from the surrounding mesoblast of the jaw. Each ingrowth of the enamel organ forms an enamel cap, which gradually embraces the dental papilla, and at the same time appears to be pushed on one side, owing to the growth not being uniform. The external layer of the dental papilla is composed of long nucleated cells, the odontoblasts, and it is by these that the dentine is formed. Similarly the internal layer of the enamel organ is formed of columnar enamel cells, which give rise to the enamel. The mesoblastic cells surrounding the base of the tooth give rise to the cement.

Bone is in many cases exoskeletal, but it will be most conveniently described with the endoskeleton.

The scales of fish are wholly or in part mesoblastic in origin, being totally different from those of reptiles. The cycloid and ctenoid scales of Teleosteans (see p. 105) are thin plates coated with epidermis. They are sometimes bony, but as a rule are simply calcified. Ganoid scales are flat plates of bone coated with an enamel-like substance, and articulating together with a peg and socket arrangement; they are probably identical with enlarged and flattened placoid scales.

The armour plates of fossil Ganoids, Labyrinthodonts, and Dinosaurs, and of living Crocodiles, some Lizards and Armadillos, are composed of bone. They are always covered by a layer of epidermis.

The antlers of deer are also composed of bone; they will be more fully described in the chapter on mammals. It may perhaps be well to mention them here, though they really belong to the endoskeleton, being outgrowths from the frontal bones.

B. Endoskeletal structures.

I. Hypoblastic.

(a) The notochord is an elastic rod formed of large vacuolated cells, and is surrounded by a membranous sheath of mesoblastic origin. It is the primitive endoskeleton in the Chordata, all of which possess it at some period of their existence; while in many of the lower forms it persists throughout life. Even in the highest Chordata it is the sole representative of the axial skeleton for a considerable part of the early embryonic life. A simple unsegmented notochord persists throughout life in the Cephalochordata, Cyclostomata, and some Pisces, such as Sturgeons and Chimaeroids.

(b) The enamel of the pharyngeal teeth of the Salmon and many other Teleosteans is hypoblastic in origin. The epiblast of the stomodaeum, in which the other teeth are developed, passes into the hypoblast of the mesenteron in which these pharyngeal teeth are formed.

II. Mesoblastic.

The most primitive type of a mesoblastic endoskeleton consists of a membranous sheath surrounding the notochord, as in Myxine and its allies. The first stage of complication is by the development of cartilage in the notochordal sheath, as in Petromyzon. Often the cartilage becomes calcified in places, as in the vertebral centra of Scyllium and other Elasmobranchs. Lastly, the formation of bone takes place; it generally constitutes the most important of the endoskeletal structures.

Bone may be formed in two ways:—

(1) by the direct ossification of pre-existing cartilage, when it is known as cartilage bone or endochondral bone;

(2) by independent ossification in connective tissue; it is then known as membrane or dermal or periosteal bone.

With the exception of the clavicle[4] all the bones of the trunk and limbs, together with a large proportion of those of the skull, are preformed in the embryo in cartilage, and are grouped as cartilage bones; while the clavicle and most of the roofing and jaw-bones of the skull are not preformed in cartilage, being developed simply in connection with a membrane. Hence it is customary to draw a very strong line of distinction between these two kinds of bone; in reality however this distinction is often exaggerated, and the two kinds pass into one another, and as will be shown immediately, the permanent osseous tissue of many of those which are generally regarded as typical cartilage bones, is really to a great extent of periosteal origin. The palatine bone, for instance, of the higher vertebrates in general is preceded by a cartilaginous bar, but is itself almost entirely a membrane bone.

Before describing the development of bone it will be well to briefly describe the structure of adult bone and cartilage.

The commonest kind of cartilage, and that which preforms so many of the bones of the embryo, is hyaline cartilage. It consists of oval nucleated cells occupying cavities (lacunae) in a clear intercellular semitransparent matrix, which is probably secreted by the cells. Sometimes one cell is seen in each lacuna, sometimes shortly after cell-division a lacuna may contain two or more cells. The free surface of the cartilage is invested by a fibrous membrane, the perichondrium.

Bone consists of a series of lamellae of ossified substance between which are oval spaces, the lacunae, giving rise to numerous fine channels, the canaliculi, which radiate off in all directions. The lacunae are occupied by the bone cells which correspond to cartilage cells, from which if the bone is young, processes pass off into the canaliculi. It is obvious that the ossified substance of bone is intercellular in character, and corresponds to the matrix of cartilage.

Bone may be compact, or loose and spongy in character, when it is known as cancellous bone. In compact bone many of the lamellae are arranged concentrically round cavities, the Haversian canals, which in life are occupied by blood-vessels. Each Haversian canal with its lamellae forms a Haversian system. In spongy bone instead of Haversian canals there occur large irregular spaces filled with marrow, which consists chiefly of blood-vessels and fatty tissue. The centre of a long bone is generally occupied by one large continuous marrow cavity. The whole bone is surrounded by a fibrous connective tissue membrane, the periosteum.

The development of bone.

Periosteal ossification. An example of a bone entirely formed in this way is afforded by the parietal. The first trace of ossification is shown by the appearance, below the membrane which occupies the place of the bone in the early embryo, of calcareous spicules of bony matter, which are laid down round themselves by certain large cells, the osteoblasts. These osteoblasts gradually get surrounded by the matter which they secrete and become converted into bone cells, and in this way a mass of spongy bone is gradually produced. Meanwhile a definite periosteum has been formed round the developing bone, and on its inner side fresh osteoblasts are produced, and these with the others gradually render the bone larger and more and more compact. Finally, the middle layer of the bone becomes again hollowed out and rendered spongy by the absorption of part of the bony matter.

Endochondral ossification[5]. This is best studied in the case of a long bone like the femur or humerus. Such a long bone consists of a shaft, which forms the main part, and two terminal portions, which form the epiphyses, or portions ossifying from centres distinct from that forming the shaft or main part of the bone.

In the earliest stage the future bone consists of hyaline cartilage surrounded by a vascular sheath, the perichondrium.

Then, starting from the centre, the cartilage becomes permeated by a number of channels into which pass vessels from the perichondrium and osteoblasts. In this way the centre of the developing shaft becomes converted into a mass of cavities separated by bands or trabeculae of cartilage. This cartilage next becomes calcified, but as yet is not converted into true bone. The osteoblasts in connection with the cavities now begin to deposit true endochondral spongy bone, and then after a time this becomes absorbed by certain large cells, the osteoclasts, and resolved into marrow or vascular tissue loaded with fat. So that the centre of the shaft passes from the condition of hyaline cartilage to that of calcified cartilage, thence to the condition of spongy bone, and finally to that of marrow. At the same time beneath the perichondrium osteoblasts are developed which also begin to give rise to spongy bone. The perichondrium thus becomes the periosteum, and the bone produced by it, is periosteal or membrane bone. So that while a continuous marrow cavity is gradually being formed in the centre of the shaft, the layer of periosteal bone round the margin is gradually thickening, and becoming more and more compact by the narrowing down of its cavities to the size of Haversian canals. The absorption of endochondral and formation of periosteal bone goes on, till in time it comes about that the whole of the shaft, except its terminations, is of periosteal origin. At the extremities of the shaft, however, and at the epiphyses, each of which is for a long time separated from the shaft by a pad of cartilage, the ossification is mainly endochondral, the periosteal bone being represented only by a thin layer.

Until the adult condition is reached and growth ceases, the pad of cartilage between the epiphysis and the shaft continues to grow, its outer (epiphysial) half growing by the formation of fresh cartilage as fast as its inner half is encroached on by the growth of bone from the shaft. The terminal or articular surfaces of the bone remain throughout life covered by layers of articular cartilage.

Even after the adult condition is reached the bone is subject to continual change, processes of absorption and fresh formation going on for a time and tending to render the bone more compact.

Methods in which bones are united to one another.

The various bones composing the endoskeleton are united to one another either by sutures or by movable joints.

When two bones are suturally united, their edges fit closely together and often interlock, being also bound together by the periosteum.

In many cases this sutural union passes into fusion or ankylosis, ossification extending completely from one bone to the other with the obliteration of the intervening suture. This feature is especially well marked in the cranium of most birds.

The various kinds of joints or articulations[6] may be subdivided into imperfect joints and perfect joints.

In imperfect joints, such as the intervertebral joints of mammals, the two contiguous surfaces are united by a mass of fibrous tissue which allows only a limited amount of motion.

In perfect joints the contiguous articular surfaces are covered with cartilage, and between them lies a synovial membrane which secretes a viscid lubricating fluid.

The amount of motion possible varies according to the nature of the articular surfaces; these include—

a. ball and socket joints, like the hip and shoulder, in which the end of one bone works in a cup provided by another, and movements can take place in a variety of planes.

b. hinge joints, like the elbow and knee, in which as in ball-and-socket joints one bone works in a cup provided by another, but movements can take place in one plane only.

THE ENDOSKELETON.

The endoskeleton is divisible into axial and appendicular parts; and the axial skeleton into—

1. the spinal column,

2. the skull {a. the cranium,
{b. the jaws and visceral skeleton,

3. the ribs and sternum[7].

I. The Axial Skeleton.

1. The Spinal column.

The spinal column in the simplest cases consists of an unsegmented rod, the notochord, surrounded by the skeletogenous layer, a sheath of mesoblastic origin, which also envelops the nerve cord. Several intermediate stages connect this simple spinal column with the vertebral column characteristic of higher vertebrates. A typical vertebral column may be said to consist of (1) a series of cartilaginous or bony blocks, the vertebral centra, which arise in the sheath surrounding the notochord. They cause the notochord to become constricted and to atrophy to a varying extent, though a remnant of it persists, either permanently or for a long period, within each centrum or between successive centra. (2) From the dorsal surface of each centrum arise a pair of processes which grow round the spinal cord and unite above it, forming a dorsal or neural arch. (3) A similar pair of processes arising from the ventral surface of the centrum form the ventral or haemal arch. To the ventral arch the ribs strictly belong, and it tends to surround the ventral blood-vessels and the body cavity with the alimentary canal and other viscera.

A neural spine or spinous process commonly projects upwards from the dorsal surface of the neural arch, and a pair of transverse processes project outwards from its sides. When, as is commonly the case, the two halves of the haemal arch do not meet, the ventral surface of the centrum often bears a downwardly-projecting hypapophysis.

The character of the surfaces by which vertebral centra articulate with one another varies much. Sometimes both surfaces are concave, and the vertebra is then said to be amphicoelous; sometimes a centrum is convex in front and concave behind, the vertebra is then opisthocoelous, sometimes concave in front and convex behind, when the vertebra is procoelous. Again, in many vertebrae both faces of the centra are flat, while in others they are saddle-shaped, as in the neck vertebrae of living birds, or biconvex, as in the case of the first caudal vertebra of crocodiles.

In the higher vertebrates pads of fibrocartilage—the intervertebral discs—are commonly interposed between successive centra, these or parts of them often ossify, especially in the trunk and tail, and are then known as inter centra.

Fig. 2. Cervical vertebrae of an Ox (Bos taurus).
A, is the fifth; B, the fourth; C, the third. X ¼ (Camb. Mus.)

1. neural spine.	6. prezygapophysis.
2. transverse process.	7. postzygapophysis.
3. hypapophysis.	8. vertebrarterial canal.
4. convex anterior face of the centrum.	9. neural canal.
5. concave posterior face of the centrum.	10. inferior lamella of transverse process.

The vertebrae of the higher forms can generally be arranged in the following five groups, each marked by certain special characteristics:

1. The cervical or neck vertebrae. These connect the skull with the thorax, and are characterised by relatively great freedom of movement. They often bear small ribs, but are distinguished from the succeeding thoracic vertebrae by the fact that their ribs do not reach the sternum. The first cervical vertebra which articulates with the skull is called the atlas, but a study of the nerve exits shows that the first vertebra is not serially homologous throughout the Ichthyopsida, so that it is best to reserve the term atlas for the first vertebra in Sauropsida and Mammalia.

2. The thoracic vertebrae (often called dorsal) bear movably articulated ribs which unite ventrally with the sternum.

3. The lumbar vertebrae are generally large, and are often more movable on one another than are the thoracic vertebrae. They bear no ribs.

4. The sacral vertebrae are characterised by the fact that they are firmly fused together, and are united with the pelvic girdle by means of their transverse processes and rudimentary ribs.

5. The caudal or tail vertebrae succeed the sacral. The anterior ones are often fused with one another and with the sacrals, but they differ from true sacral vertebrae in that there are no rudimentary ribs between their transverse processes and the pelvic girdle. They often bear V-shaped chevron bones.

In fish and snakes the vertebral column is divisible into only two regions, an anterior trunk region, whose vertebrae bear ribs, and a posterior tail region, whose vertebrae are ribless.

2. The Skull.

Before giving a general account of the adult skull it will be well to briefly describe its development.

General development of the Cranium[8].

Shortly after its appearance, the central nervous system becomes surrounded by a membranous mesodermal investment which in the region of the spinal cord is called the skeletogenous layer or perichordal sheath, while in the region of the brain it is called the membranous cranium. Ventral to the central nervous system is the notochord, which extends far into the region of the future cranium, and like the nervous system, is enclosed by the skeletogenous layer. The primitive cartilaginous cranium is formed by histological differentiation within the substance of the membranous cranium and always consists of the following parts:

(a) the parachordals. These are a pair of flat curved plates of cartilage, each of which has its inner edge grooved where it comes in contact with the notochord. The parachordals, together with the notochord, form a continuous plate, which is known as the basilar plate. The basilar plate is the primitive floor below the hind- and mid-brain. In front the parachordals abut upon another pair of cartilaginous bars, the trabeculae, the two pairs of structures being sometimes continuous with one another from the first;

(b) the trabeculae which meet behind and embrace the front end of the notochord. Further forwards they at first diverge from one another, and then converge again, enclosing a space, the pituitary space. After a time they generally fuse with one another in the middle line, and, with the parachordals behind, form an almost continuous basal plate. The trabeculae generally appear before the parachordals. They form the primitive floor below the fore-brain;

(c) the cartilaginous capsules of the three pairs of sense organs. At a very early stage of development involutions of the surface epiblast give rise to the three pairs of special sense organs—the olfactory or nasal organs in front, the optic in the middle, and the auditory behind. The olfactory and auditory organs always become enclosed in definite cartilaginous capsules, the eyes often as in the Salmon, become enclosed in cartilaginous sclerotic capsules, while sometimes, as in mammals, their protecting capsules are fibrous.

Each pair of sense capsules comes into relation with part of the primitive cranium, and greatly modifies it. Thus the auditory or periotic capsules press on the parachordals till they come to be more or less imbedded in them. Perhaps owing to the pressure of the nasal capsules the trabeculae fuse in front, and then grow out into an anterior pair of processes, the cornua trabeculae, and a posterior pair, the antorbital processes, which together almost completely surround the nasal capsules. The sclerotic capsules of the eyes greatly modify the cranium, although they never become completely united with it.

The cartilaginous cranium formed of the basal plate, together with the sense capsules, does not long remain merely as a floor. Its sides grow vertically upwards, forming the exoccipital region of the cranium behind, and the alisphenoidal and orbitosphenoidal regions further forwards. In many forms, such as Elasmobranchs, all these upgrowths meet round the brain, roofing it in and forming an almost complete cartilaginous cranium. But in most vertebrata, while in the occipital region, the cartilaginous cranium is completed dorsally, in the alisphenoidal and orbitosphenoidal regions the cartilage merely forms the lateral walls of the cranium, the greater part of the brain having dorsal to it a wide space, closed by merely membranous tissue in connection with which the large frontal and parietal bones are subsequently formed.

The Skull includes

a. the cranium,

b. the jaws and visceral skeleton.

The cranium can be further subdivided into

(1) an axial portion, the cranium proper or brain case;

(2) the sense capsules. The capsules of the auditory and olfactory sense organs are always present, and as has been already mentioned, in many animals the eye likewise is included in a cartilaginous capsule.

(1) The cranium proper or brain case.

The cranium varies much in form and structure. In lower vertebrates, such as Sharks and Lampreys, it remains entirely cartilaginous and membranous, retaining throughout life much of the character of the embryonic rudiment of the cranium of higher forms. The dogfish's cranium, described on pp. 73 to 76, is a good instance of a cranium of this type. But in the majority of vertebrates the cartilage becomes more or less replaced by cartilage bone, while membrane bones are also largely developed and supplant the cartilage.

The cranium of most vertebrates includes a very large number of bones whose arrangement varies much, but one can distinguish a definite basicranial axis formed of the basi-occipital, basisphenoid, and presphenoid bones, which is a continuation forwards of the axis of the vertebral column. From the basicranial axis a wide arch arises, composed of a number of bones, which form the sides and roof of the brain-case These bones are arranged in such a manner that if both cartilage and membrane bones are included they can be divided into three rings or segments. The hinder one of these segments is the occipital, the middle the parietal, and the anterior one the frontal.

The occipital segment is formed of four cartilage bones, the basi-occipital below, two exoccipitals at the sides, and the supra-occipital above. The parietal segment is formed of the basisphenoid below, two alisphenoids at the sides and two membrane bones, the parietals above, and the frontal segment in like manner consists of the presphenoid below, the two orbitosphenoids at the sides, and two membrane bones, the frontals, above. The parietals and frontals, being membrane bones, are not comparable to the supra-occipital, in the way that the presphenoid and basisphenoid are to the basi-occipital.

The cartilage bones of the occipital segments are derived from the parachordals of the embryonic skull, those of the parietal and frontal segments from the trabeculae.

In front of the presphenoid the basicranial axis is continued by the mesethmoid.

(2) The sense capsules.

These enclose and protect the special sense organs.

(a) Auditory capsule.

The basisphenoid is always continuous with the basi-occipital, but the alisphenoid is not continuous with the exoccipital as the periotic or auditory capsule is interposed between them. Each periotic capsule has three principal ossifications; an anterior bone, the pro-otic, a posterior bone, the opisthotic, and a superior bone, the epi-otic.

These bones may severally unite, or instead of uniting with one another they may unite with the neighbouring bones. Thus the epi-otic often unites with the supra-occipital, and the opisthotic with the exoccipital.

Two other bones developed in the walls of the auditory capsule are sometimes added, as in Teleosteans; these are the pterotic and sphenotic.

(b) Optic capsule.

The eye is frequently enclosed in a cartilaginous sclerotic capsule, and in this a number of scale-like bones are often developed.

Several membrane bones are commonly formed around the orbit or cavity for the eye. The most constant of these is the lachrymal which lies in the anterior corner; frequently too, as in Teleosteans, there is a supra-orbital lying in the upper part of the orbit, or as in many Reptiles, a postorbital lying in the posterior part of the orbit.

(c) Nasal capsule.

In relation to the nasal capsules various bones occur.

The basicranial axis in front of the presphenoid is ossified, as the mesethmoid, dorsal to which there sometimes, as in Teleosteans, occur a median ethmoid and a pair of lateral ethmoids[9]. Two pairs of membrane bones very commonly occur in this region, viz. the nasals which lie dorsal to the mesethmoid, and the vomers (sometimes there is only one) which lie ventral to it.

The part of the skull lying immediately in front of the cranial cavity and in relation to the nasal capsules constitutes the ethmoidal region.

There remain certain other membrane bones which are often found connected with the cranium. Of these, one of the largest is the parasphenoid which, in Ichthyopsids, is found underlying the basicranial axis. Prefrontals often, as in most reptiles, occur lying partly at the sides and partly in front of the frontal, and postfrontals similarly occur behind the orbit lying partly behind the frontals and partly at their sides. Lastly a squamosal bone is, as in Mammals, very commonly developed, and lies external and partly dorsal to the auditory capsules.

The Jaws and Visceral Skeleton.

In the most primitive fish these consist of a series of cartilaginous rings or arches placed one behind another and encircling the anterior end of the alimentary canal. Originally they are mainly concerned with branchial respiration.

The first or maxillo-mandibular arch forms the upper jaw and the lower jaw or mandible.

The second or hyoid arch bears gills and often assists in attaching the jaws to the cranium. The remaining arches may bear gills, though the last is commonly without them.

The above condition is only found in fishes, in higher animals the visceral skeleton is greatly reduced and modified.

The first or maxillo-mandibular arch is divisible into a dorsal portion, the palato-pterygo-quadrate bar, which forms the primitive upper jaw and enters into very close relations with the cranium, and a ventral portion, Meckel's cartilage, which forms the primitive lower jaw. The cartilaginous rudiments of both these portions disappear to a greater or less extent and become partly ossified, partly replaced by or enveloped in membrane bone.

The posterior part of the palato-pterygo-quadrate bar becomes ossified to form the quadrate, the anterior part to form the palatine and pterygoid, or the two latter may be formed partially or entirely of periosteal bone, developed round the cartilaginous bar. Two pairs of important membrane bones, the premaxillae and maxillae form the anterior part of the upper jaw, and behind the maxilla lies another membrane bone, the jugal or malar, which is connected with the quadrate by a quadratojugal. The premaxillae have a large share in bounding the external nasal openings or anterior nares.

In lower vertebrates the nasal passage leads directly into the front part of the mouth cavity and opens by the posterior nares. In some higher vertebrates, such as mammals and crocodiles, processes arise from the premaxillae and palatines, and sometimes from the pterygoids, which meet their fellows in the middle line and form the palate, shutting off the nasal passage from the mouth cavity and causing the posterior nares to open far back.

The cartilage of the lower jaw is in all animals with ossified skeletons, except the Mammalia, partly replaced by cartilage bone forming the articular, partly overlain by a series of membrane bones the dentary, splenial, angular, supra-angular and coronoid. In many sharks large paired accessory cartilages occur at the sides of the jaws; and in a few reptiles and some Amphibia, such as the Frog, the ossified representative of the anterior of these structures occurs forming the mento-meckelian bone. In mammals the lower jaw includes but a single bone.

The quadrate in all animals with ossified skeletons, except the Mammalia, forms the suspensorium of the mandible or the skeletal link between the jaw and the cranium; in the Mammalia, however, the mandible articulates with the squamosal, while the quadrate is greatly reduced, and is now generally considered to be represented by the tympanic ring of the ear.

The second visceral or hyoid arch in fishes consists of two pieces of cartilage, a proximal piece the hyomandibular, and a distal[10] piece the cerato-hyal. The cerato-hyals of the two sides are commonly united by a median ventral plate, the basi-hyal. The hyoid arch bears gills on its posterior border, but its most important function in most fishes is to act as the suspensorium. In higher vertebrates the representative of the hyomandibular is much reduced in size, and comes into relation with the ear forming the auditory ossicles; the cerato-hyal looses its attachment to the hyomandibular and becomes directly attached to the cranium, forming a large part of the hyoid apparatus of most higher vertebrates.

Behind the hyoid arch come the branchial arches. They are best developed in fishes, in which they are commonly five in number and bear gills. Their ventral ends are united in pairs by median pieces, the copulae.

In higher vertebrates they become greatly reduced, and all except the first and second completely disappear. In the highest vertebrates, the mammals, the second has disappeared, but in birds and many reptiles it is comparatively well developed.

3. The Ribs and Sternum.

The ribs are a series of segmentally arranged cartilaginous or bony rods, attached to the vertebrae; they tend to surround the body cavity, and to protect the organs contained within it. Ribs are very frequently found attached to the transverse processes of the vertebrae, but a study of their origin in fish shows that they are really the cut off terminations of the ventral arch, not of the transverse processes which are outgrowths from the dorsal arch. In the tail their function is to surround and protect structures like the ventral blood-vessels which do not vary much in size, consequently they meet one another, and form a series of complete ventral or haemal arches. But the trunk contains organs like the lungs and stomach which are liable to vary much in size at different times, consequently the halves of the haemal arch do not meet ventrally, and then the ribs become detached from the rest of the haemal arch. Having once become detached, they are able to shift about and unite themselves to various points of the vertebra. They frequently, as has been already mentioned, become entirely attached to the transverse process, or they may be attached to the transverse process by a dorsal or tubercular portion and to the centrum or to the ventral arch by a ventral or capitular portion.

In all animals above fishes the distal ends of the thoracic ribs unite with a median breast bone or sternum which generally has the form of a segmented rod. The sternum is really formed by the fusion of the distal ends of a series of ribs. In many animals elements of the shoulder girdle enter into close relation with the rib elements of the sternum.

II. The Appendicular Skeleton.

This consists of the skeleton of the anterior or pectoral, and the posterior or pelvic limbs, and their girdles. In every case (except in Chelonia) the parts of the appendicular skeleton lie external to the ribs.

1. The Limb girdles.

The Pectoral girdle[11]. In the simplest case the pectoral or shoulder girdle consists of a hoop of cartilage incomplete dorsally. It is attached by muscle to the vertebral column, and is divided on either side into dorsal and ventral portions by a cavity, the glenoid cavity, at the point where the anterior limb articulates. In higher fishes this hoop is distinctly divided into right and left halves; it becomes more or less ossified, and a pair of important bones, the clavicles, are developed in connection with its ventral portion.

In higher vertebrates ossification sets up in the cartilage and gives rise on each side to a dorsal bone, the scapula, and frequently to an anterior ventral bone, the precoracoid, and a posterior ventral bone, the coracoid. The precoracoid is often not ossified, and upon it is developed the clavicle which more or less replaces it. In some forms a T shaped interclavicle occurs, in others epicoracoids are found in front of the coracoids. In all vertebrata above fish, except the great majority of mammals, the coracoids are large and articulate with the sternum. But in mammals the coracoids are nearly always quite vestigial, and the pectoral girdle is attached to the axial skeleton by the clavicle or sometimes by muscles and ligaments only.

The Pelvic girdle[12] like the pectoral consists primitively of a simple rod or hoop of cartilage, which in vertebrata above fishes is divided into dorsal and ventral portions, by a cavity, the acetabulum, with which the posterior limb articulates. In the pelvic girdle as in the pectoral one dorsal, and (commonly) two ventral ossifications take place. The dorsal bone is the ilium and corresponds to the scapula. The posterior ventral bone is the ischium corresponding to the coracoid. The anterior ventral bone is the pubis and is generally compared to the precoracoid, but in some cases a fourth pelvic element, the acetabular or cotyloid bone is found, and this may correspond to the precoracoid.

The pelvic girdle differs from the pectoral in the fact that the dorsal bones—the ilia—are nearly always firmly united to transverse processes of the sacral vertebrae, by means of rudimentary ribs. The pubes and ischia generally meet in ventral symphyses.

2. The Limbs.

It will be most convenient to defer a discussion of the limbs of fishes to chap. VIII.

All vertebrates above fishes have the limbs divisible into three main segments:—

The proximal segments each contain one bone, the humerus in the case of the upper arm, and the femur in the case of the thigh. The middle segments each contain two bones, the radius and ulna in the case of the fore-arm, and the tibia and fibula in the case of the shin.

The manus and pes are further subdivided into

(a) two or three proximal rows of bones forming the wrist or carpus in the case of the manus, and the ankle or tarsus in the case of the pes.

(b) a middle row called respectively the metacarpus and metatarsus.

(c) a number of distal bones called the phalanges which form the skeleton of the fingers and toes, or digits.

Typically the manus and pes both have five digits (pentedactylate). The first digit of the manus is commonly called the pollex, and the first digit of the pes the hallux.

In a very simple carpus such as that of Chelydra, there are nine bones. They are arranged in a proximal row of three, the radiale, intermedium, and ulnare,—the first being on the radial side of the limb, and a distal row of five called respectively carpale 1, 2, 3, 4, 5, beginning on the radial side. Between these two rows is a single bone the centrale, or there may be two.

Similarly there are nine bones in a simple tarsus such as that of Salamandra. They form a proximal row of three, the tibiale, intermedium and fibulare, and a distal row of five, called respectively tarsale 1, 2, 3, 4, 5, beginning on the tibial side. Between the two rows there is a centrale as in the carpus, or there may be two.

The following names derived from human anatomy are commonly applied to the various carpal and tarsal bones:

Note. The above is the view commonly accepted concerning the homology of the carpal and tarsal bones. But with regard to the proximal row of tarsal bones there is difference of opinion. All anatomists are agreed that the calcaneum is the fibulare and that the intermedium is contained in the astragalus, but while the majority regard the astragalus as the fused tibiale and intermedium, Baur considers that a small bone found on the tibial side of the tarsus in Procavia, many Rodents, Insectivores, and the male Ornithorhynchus, is the vestigial tibiale, and regards the astragalus as the intermedium alone[13]. He also considers that the mammalian scaphoid represents a centrale.

Modifications in the positions of the limbs[14].

In their primitive position the limbs are straight and are extended parallel to one another at right angles to the axis of the trunk. Each limb then has a dorsal surface, a ventral surface, an anterior or pre-axial edge, and a posterior or postaxial edge.

In the anterior limb the radius and the pollex are pre-axial, the ulna and the fifth finger are postaxial. In the posterior limb the tibia and the hallux are pre-axial, the fibula and the fifth toe are postaxial. The Cetacea and various extinct reptiles, such as Ichthyosaurus and Plesiosaurus, have their limbs in practically this primitive position.

The first modification from it is produced by the bending ventrally of the middle segments of both limbs upon the proximal segments, while the distal segment is bent in the opposite direction on the middle segment. Then the ventral surfaces of the antibrachium and crus come to look inwards, and their dorsal surfaces to look outwards. The brachium and manus, thigh and pes still have their dorsal surfaces facing upwards and their ventral surfaces facing downwards as before, and the relations of their pre- and postaxial borders remain as they were. Many Amphibians and Reptiles, such as tortoises, carry their limbs in this position.

In all higher vertebrates, however, a further change takes place, each limb is rotated as a whole from its proximal end, the rotation taking place in opposite directions in the fore and hind limbs respectively. The anterior limb is rotated backwards from the shoulder, so that the brachium lies nearly parallel to the body, and the elbow points backwards, the antibrachium downwards, and the manus backwards; the pre-axial surface of the whole limb with the radius and pollex now faces outwards, and the postaxial surface with the ulna and fifth finger now faces inwards. In the Walrus and, to a certain extent, in the Sea lions the anterior limb remains throughout life in this position. The posterior limb is also rotated, but the rotation in this case takes place forwards, so that the thigh lies nearly parallel to the body, the knee-joint pointing forwards; the crus downwards and the pes forwards. The pre-axial surface of the whole limb with the tibia and hallux looks towards the middle of the body, the postaxial surface with the fibula and fifth toe looks outwards. This is the position in which the hind limb is carried in nearly all mammals.

In nearly all mammals a further change takes place in the position of the anterior limb. The radius and ulna have hitherto been parallel to one another, but now the lower end of the radius, carrying with it the manus, comes to be rotated forwards round the ulna, so that the manus, as well as the pes, comes to be forwardly-directed, and its pre-axial surface faces inwards.

In the majority of mammals the radius and ulna are permanently fixed in this, which is known as the prone position, but in man and some other mammals the manus can be pronated or turned into this position at will. When the radius and ulna are parallel throughout their whole length the manus is said to be in the supine position.

The extensor side of a limb is that to which the muscles which straighten it are attached, the flexor side is that to which the muscles which bend it are attached.

The following classification includes only the forms mentioned in the succeeding pages. The relative value of some of the terms employed in classification is not identical throughout the book. This remark applies specially to the term group, which is a convenient one, owing to its not having such a hard and fast zoological meaning as has the term family, for instance. The term group is applied in this book to divisions of the animal kingdom of very different classificatory importance.

PHYLUM CHORDATA.

SUBPHYLUM A. HEMICHORDATA.

• Balanoglossus.
• Cephalodiscus.
• Rhabdopleura.
• ? Phoronis.
• (? Actinotrocha—larval Phoronis).

SUBPHYLUM B. UROCHORDATA (TUNICATA).

• Group Larvacea and others.

SUBPHYLUM C. CEPHALOCHORDATA.

• Amphioxus—lancelet.

Note. In this chapter all the generic names printed in italics are those of extinct animals.

SUBPHYLUM D. VERTEBRATA.

DIVISION (I). CYCLOSTOMATA.

• Order 1. Marsipobranchii.
• Family Myxinoidei. Myxine—hag-fish.
• Bdellostoma.
• Family Petromyzontidae. Petromyzon—lamprey.
• (Ammocoetes—larval lamprey.)
• Family Palaeospondylidae. Palaeospondylus.
• Order 2. Ostracodermi.
• Suborder 1. Heterostraci.
• Family Pteraspidae. Pteraspis.
• Suborder 2. Osteostraci.
• Family Cephalaspidae. Cephalaspis.
• Suborder 3. Antiarcha.
• Family Asterolepidae. Pterichthys.
• Asterolepis.

DIVISION (II). GNATHOSTOMATA.

A. ICHTHYOPSIDA.

CLASS I. PISCES.

• Order 1. Elasmobranchii.
• Suborder (1). Ichthyotomi.
• Family Pleuracanthidae. Xenacanthus.
• Suborder (2). Pleuropterygii.
• Cladoselache.
• Suborder (3). Selachii.
• Group Squalidae.
• Family Notidanidae. Heptanchus.
• Hexanchus.
• Chlamydoselache—frill-gilled shark.
• Family Cochliodontidae. Cochliodus.

• Family Cestraciontidae. Cestracion—Port Jackson shark.
• Acrodus.
• Family Scylliidae. Scyllium—spotted dogfish.
• Family Lamnidae. Odontaspis.
• Family Carcharidae. Galeus—tope.
• Family Spinacidae. Acanthias—spiny dogfish.
• Scymnus.
• Family Squatinidae. Squatina (Rhina)—angel fish.
• Group Batoidei.
• Family Pristidae. Pristis—saw-fish.
• Family Raiidae. Raia—skate.
• Family Myliobatidae. Myliobatis—eagle ray.
• Family Trygonidae. Trygon—sting ray.
• Family Torpedinidae. Torpedo—electric ray.
• Suborder (4). Acanthodii.
• Family Acanthodidae. Acanthodes.
• Family Diplacanthidae. Diplacanthus.
• Order 2. Holocephali.
• Family Chimaeridae. Chimaera—rabbit fish.
• Harriotta.
• Callorhynchus.
• Ischyodus.
• Order 3. Ganoidei.
• Suborder (1). Chondrostei.
• Family Palaeoniscidae. Palaeoniscus.
• Trissolepis.
• Family Acipenseridae. Acipenser—sturgeon.
• Scaphirhynchus.
• Family Polyodontidae. Polyodon (Spatularia)—spoon-beaked sturgeon.
• Psephurus—slender-beaked sturgeon.

• Suborder (2). Crossopterygii.
• Family Holoptychiidae. Holoptychius.
• Family Rhizodontidae. Rhizodus.
• Family Osteolepidae. Osteolepis.
• Family Polypteridae. Polypterus—bichir.
• Calamoichthys—reed-fish.
• Suborder (3). Holostei.
• Family Lepidosteidae. Lepidosteus—gar pike.
• Family Semionotidae. Lepidotus.
• Family Amiidae. Amia—bow-fin.
• Order 4. Teleostei.
• Suborder (1). Plectognathi.
• Family Balistidae. Balistes—file-fish.
• Family Gymnodontidae. Diodon—globe-fish.
• Family Ostracionidae. Ostracion—coffer-fish.
• Suborder (2). Physostomi.
• Family Siluridae.—cat-fishes.
• Family Cyprinidae. Cyprinus—carp.
• Family Esocidae. Esox—pike.
• Family Salmonidae. Salmo—salmon.
• Family Clupeidae. Clupeus—herring.
• Exocaetus—'flying fish'.
• Family Muraenidae. Anguilla—eel.
• Suborder (3). Anacanthini.
• Family Gadidae. Gadus—cod, haddock, whiting.
• Family Pleuronectidae. Solea—sole.
• Suborder (4). Pharyngognathi.
• Family Labridae. Labrus—wrasse.
• Scarus—parrot fish.

• Suborder (5). Acanthopterygii.
• Family Cataphracti. Dactylopterus—flying gurnard.
• Family Percidae. Perca—perch.
• Order 5. Dipnoi.
• Suborder (1). Sirenoidei.
• Family Dipteridae. Dipterus.
• Family Monopneumona. Ceratodus—barramunda.
• Family Dipneumona. Protopterus—African mud-fish.
• Lepidosiren.
• Suborder (2). Arthrodira.
• Family Coccosteidae. Coccosteus.
• Dinichthys.

Note. Palaeontological research has disclosed the existence of a great number of forms which seem to connect with one another almost all the orders of fishes as usually recognised. Forms connecting the living Ganoids with the Teleosteans have been especially numerous, so that these terms Ganoid and Teleostean can hardly be any longer used in a precise and scientific sense. This has rendered the subject of the classification of fishes a very difficult one. Though unsuitable for adoption in a work like the present, by far the most natural classification hitherto proposed seems to be that of Smith Woodward[15]. He considers that the course of development of fishes has followed two distinct lines, the autostylic and hyostylic (see p. 119), and groups the various forms as follows:

Hyostylic.	Autostylic.
Subclass 1. Elasmobranchii.	Subclass 3. Holocephali.
1. Ichthyotomi.	1. (unknown).
2. Selachii.	2. Chimaeroidei.
3. Acanthodii.	3. (unknown).
Subclass 2. Teleostomi.	Subclass 4. Dipnoi.
1. Crossopterygii (Palaeozoic	1. Sirenoidei.
and Mesozoic).
2. Crossopterygii (Cainozoic).	2. (unknown).
3. Actinopterygii.	3. Arthrodira.

The primitive forms in each of these four subclasses have the fins archipterygia (see p. 127).

CLASS II. AMPHIBIA.

• Order 1. Urodela.
• Suborder (1). Ichthyoidea.
• Group A. ,Perennibranchiata.
• Family Menobranchidae. Menobranchus.
• Family Proteidae. Proteus—olm.
• Family Sirenidae. Siren.
• Group B. Derotremata.
• Family Amphiumidae. Megalobatrachus.
• Cryptobranchus (Menopoma).
• Amphiuma.
• Suborder (2). Salamandrina.
• Family Salamandridae. Salamandra—salamander.
• Molge—newt.
• Onychodactylus.
• Amblystoma.
• (Siredon—axolotl, larval Amblystoma).
• Batrachoseps.
• Spelerpes (Gyrinophilus).
• Order 2. Labyrinthodontia.
• Group Lepospondyli. Branchiosaurus.
• Group Temnospondyli. Archegosaurus.
• Nyrania.
• Euchirosaurus.
• Group Stereospondyli. Capitosaurus.
• Mastodonsaurus.
• Order 3.Gymnophiona.
• Family Caeciliidae. Siphonops.
• Epicrium.

• Order 4. Anura.
• Suborder (1). Aglossa.
• Family Xenopidae. Xenopus.
• Family Pipidae. Pipa—Surinam toad.
• Suborder (2). Phaneroglossa.
• Group Arcifera.
• Family Discoglossidae. Discoglossus—painted frog.
• Bombinator—fire-bellied frog.
• Alytes—midwife frog.
• Family Pelobatidae. Pelobates—toad frog.
• Family Hylidae. Hyla—green tree-frog.
• Family Bufonidae. Bufo—toad.
• Docidophryne.
• Family Cystignathidae. Ceratophrys—horned frog.
• Group Firmisternia.
• Family Ranidae. Rana—common and edible frogs.
• Family Engystomatidae. Brachycephalus.

B. SAUROPSIDA.

CLASS I. REPTILIA[16].

• Order 1. Theromorpha.
• Group Anomodontia. Dicynodon.
• Udenodon.
• Group Placodontia. Placodus.
• Group Pariasauria. Pariasaurus.
• Elginia.
• Group Theriodontia. Dimetrodon.
• Galesaurus.
• Cynognathus.

• Order 2. Sauropterygia.
• Family Mesosauridae. Mesosaurus.
• Family Nothosauridae. Nothosaurus.
• Family Plesiosauridae. Plesiosaurus.
• Pliosaurus.
• Order 3. Chelonia.
• Suborder (1). Trionychia.
• Family Trionychidae. Trionyx—snapping turtle.
• Suborder (2). Cryptodira.
• Family Dermochelydidae. Dermochelys (Sphargis)—leathery
• turtle.
• Family Chelonidae. Chelone—green turtle.
• Family Chelydridae. Chelydra—terrapin.
• Family Chersidae. Testudo—tortoise.
• Suborder (3). Pleurodira.
• Family Chelydae. Chelys.
• Order 4. Ichthyosauria.
• Family Ichthyosauridae. Ichthyosaurus.
• Order 5. Rhynchocephalia.
• Suborder (1). Rhynchocephalia vera.
• Family Sphenodontidae. Sphenodon (Hatteria).
• Family Rhynchosauridae. Hyperodapedon.
• Suborder (2). Proganosauria.
• Family Proterosauridae. Proterosaurus.
• Order 6. Squamata.
• Suborder (1). Lacertilia.
• Group Lacertilia vera.
• Family Geckonidae. Gecko.
• Family Pygopodidae. Lialis—scale-foot.

• Family Agamidae. Draco—flying lizard.
• Agama.
• Family Iguanidae. Iguana.
• Family Anguidae. Ophisaurus (Bipes, Pseudopus).
• Anguis—blindworm.
• Family Varanidae. Varanus—monitor.
• Family Amphisbaenidae. Chirotes.
• Amphisbaena.
• Family Scincidae. Tiliqua (Cyclodus).
• Scincus—skink.
• Chalcides (Seps).
• Group Rhiptoglossa.
• Family Chamaeleonidae. Chamaeleon.
• Suborder (2). Ophidia.
• Family Typhlopidae. Typhlops—blind snake.
• Family Boidae. Python.
• Family Colubridae. Tropidonotus—ringed snake.
• Family Hydrophidae—sea snakes.
• Family Crotalidae. Crotalus—rattlesnake.
• Suborder (3). Pythonomorpha.
• Family Mosasauridae. Mosasaurus.
• Order 7. Dinosauria.
• Suborder (1). Sauropoda.
• Family Atlantosauridae. Brontosaurus.
• Family Cetiosauridae. Morosaurus.
• Suborder (2). Theropoda.
• Family Megalosauridae. Megalosaurus (Ceratosaurus).
• Family Compsognathidae. Compsognathus.

• Suborder (3). Orthopoda.
• Section (a). Stegosauria.
• Family Scelidosauridae. Polacanthus.
• Family Stegosauridae. Stegosaurus.
• Section (b). Ceratopsia.
• Family Ceratopsidae. Polyonax (Ceratops).
• Section (c). Ornithopoda.
• Family Camptosauridae. Hypsilophodon.
• Family Iguanodontidae. Iguanodon.
• Family Hadrosauridae. Hadrosaurus.
• Order 8. Crocodilia.
• Suborder (1). Parasuchia.
• Family Phytosauridae. Phytosaurus (Belodon).
• Suborder (2). Eusuchia.
• Family Teleosauridae. Teleosaurus.
• Metriorhynchus.
• Family Goniopholidae. Goniopholis.
• Family Alligatoridae. Alligator.
• Caiman.
• Jacare.
• Family Crocodilidae. Crocodilus.
• Family Garialidae. Garialis (Gavialis).
• Order 9. Pterosauria.
• Family Pterodactylidae. Pterodactylus.
• Family Rhamphorhynchidae. Rhamphorhynchus.
• Family Pteranodontidae. Pteranodon.

CLASS II. AVES[17].

• Subclass (I). Archaeornithes.
• Archaeopteryx.
• Subclass (II). Neornithes.
• Order 1. Ratitae.
• Group Æpyornithes. Æpyornis.
• Group Apteryges. Apteryx—kiwi.
• Group Dinornithes. Moas.
• Group Megistanes. Casuarius—cassowary.
• Dromaeus—emeu.
• Group Rheornithes. Rhea—American ostrich.
• Group Struthiornithes. Struthio—ostrich.
• Order 2. Odontolcae.
• Hesperornis.
• Order 3. Carinatae.
• Group Ichthyornithiformes.
• Ichthyornis.
• Apatornis.
• Odontopteryx.
• Group Colymbiformes.
• Subgroup Colymbi—divers.
• Group Sphenisciformes.
• Subgroup Sphenisci—penguins.

• Group Ciconiiformes.
• Subgroup Steganopodes. Sula—gannet.
• Pelicanus—pelican.
• Phaëthon—frigate bird.
• Phalacrocorax—cormorant.
• Subgroup Ardeae. Ardea—heron
• Subgroup Ciconiae. Leptoptilus—adjutant.
• Ciconia—white stork.
• Group Anseriformes.
• Subgroup Palamedeae. Palamedea }
• } screamers.
• Chauna    }
• Subgroup Anseres. Anas—wild duck.
• Anser—goose.
• Plectropterus—spur-winged goose.
• Cygnus—swan.
• Mergus—merganser.
• Group Falconiformes.
• Subgroup Cathartae. Cathartes—American vulture.
• Subgroup Accipitres. Falco—falcon.
• Vultur—vulture.
• Harpagus.
• Gypogeranus—secretary bird.
• Group Tinamiformes.
• Subgroup Tinami. Tinamus.
• Group Galliformes.
• Subgroup Galli. Gallus—fowl.
• Pavo—peacock.
• Subgroup Opisthocomi. Opisthocomus—hoatzin.
• Group Gruiformes.
• Gruidae—cranes.
• Group Stereornithes. Phororhacos.

• Group Charadriiformes.
• Subgroup Limicolae. Charadriidae—plovers.
• Parra—jacana.
• Subgroup Lari. Laridae—gulls.
• Alcidae—auks.
• Subgroup Pteroclidae. Pterocles—sandgrouse.
• Subgroup Columbidae. Columbae—pigeons.
• Didus—dodo.
• Pezophaps—solitaire.
• Group Cuculiformes.
• Subgroup Cuculi. Scythrops.
• Subgroup Psittaci. Stringops—owl-parrot.
• Group Coraciiformes.
• Subgroup Coraciae. Coracias—roller.
• Buceros—hornbill.
• Upupa—hoopoe.
• Subgroup Striges. Owls.
• Subgroup Cypseli. Cypselidae—swifts.
• Trochilidae—humming-birds.
• Subgroup Trogonidae. Trogons.
• Subgroup Pici. Rhamphastos—toucan.
• Picus—woodpecker.
• Group Passeriformes. Crows, finches, larks, warblers,
• and many others.

C. MAMMALIA[18].

Class MAMMALIA.

• Subclass (I). Ornithodelphia or Prototheria.
• Order. Monotremata.

• Family Ornithorhynchidae. Ornithorhynchus—duck-bill.
• Family Echidnidae. Echidna—spiny ant-eater.
• Group Multituberculata. Tritylodon.
• Subclass (II). Didelphia or Metatheria.
• Order. Marsupialia.
• Suborder (1). Polyprotodontia.
• Family Amphitheriidae. Phascolotherium.
• Family Didelphyidae. Didelphys—opossum.
• Family Dasyuridae. Thylacinus—Tasmanian wolf.
• Sarcophilus—Tasmanian devil.
• Dasyurus.
• Family Peramelidae. Perameles—bandicoot.
• Choeropus.
• Family Notoryctidae. Notoryctes—marsupial mole.
• Suborder (2). Diprotodontia.
• Family Phascolomyidae. Phascolomys—wombat.
• Family Phalangeridae. Tarsipes.
• Phalanger—cuscus.
• Phascolarctus—koala.
• Thylacoleo.
• Family Diprotodontidae. Diprotodon.
• Family Nototheriidae. Nototherium.
• Family Macropodidae. Macropus—kangaroo.
• Family Epanorthidae. Coenolestes.
• Subclass (III). Monodelphia or Eutheria.
• Order 1. Edentata.
• Family Bradypodidae. Bradypus }
• }—sloths.
• Choloepus }

• Family Megatheriidae. Megatherium—ground sloth.
• Family Myrmecophagidae. Myrmecophaga—great ant-eater.
• Cycloturus—two-toed ant-eater.
• Family Dasypodidae. Chlamydophorus }
• Dasypus }—armadillos.
• Priodon }
• Tatusia }
• Family Glyptodontidae. Glyptodon.
• Family Manidae. Manis—pangolin.
• Family Orycteropodidae. Orycteropus—aard vark.
• Order 2. Sirenia.
• Family Manatidae. Manatus—manatee.
• Family Rhytinidae. Rhytina—Steller's sea-cow.
• Family Halicoridae. Halicore—dugong.
• Family Halitheriidae. Halitherium.
• Order 3. Cetacea.
• Suborder (1). Archaeoceti.
• Family Zeuglodontidae. Zeuglodon.
• Suborder (2). Mystacoceti or Balaenoidea.
• Family Balaenidae. Balaena—right whale.
• Megaptera—humpbacked whale.
• Balaenoptera—rorqual.
• Suborder (3). Odontoceti.
• Family Physeteridae. Physeter—sperm whale.
• Hyperoödon—bottlenose.
• Ziphius.
• Mesoplodon.
• Family Physodontidae. Physodon.

• Family Squalodontidae. Squalodon.
• Family Platanistidae. Platanista—Gangetic dolphin.
• Inia.
• Pontoporia.
• Family Delphinidae. Monodon—narwhal.
• Phocaena—porpoise.
• Orca—killer.
• Globicephalus—Ca'ing whale.
• Grampus.
• Lagenorhynchus.
• Delphinus—dolphin.
• Tursiops.
• Prodelphinus.
• Order 4. Ungulata.
• Division A. Ungulata vera.
• Suborder (1). Artiodactyla.
• Section (a). Suina.
• Family Hippopotamidae. Hippopotamus.
• Family Suidae. Sus—pig.
• Babirussa.
• Phacochaerus—wart hog.
• Hyotherium.
• Family Cotylopidae. Cotylops (Oreodon).
• Cyclopidius.
• Family Agriochoeridae. Agriochoerus.
• Family Anoplotheriidae. Anoplotherium.
• Section (b). Tylopoda.
• Family Camelidae. Camelus—camel.
• Auchenia—llama.
• Section (c). Tragulina.
• Family Tragulidae. Dorcatherium (Hyomoschus)—chevrotain.

Section (d). Ruminantia or Pecora.

• Family Cervidae. Moschus—musk deer.
• Cervus—deer.
• Cervulus—muntjac.
• Hydropotes—Chinese water deer.
• Family Giraffidae. Giraffa—giraffe.
• Sivatherium.
• Family Antilocapridae. Antilocapra—prongbuck.
• Family Bovidae. Tetraceros—four-horned antelope.
• Gazella—gazelle.
• Bos—ox.
• Bison.
• Bubalus—buffalo.
• Suborder (2). Perissodactyla.
• Family Tapiridae. Tapirus—tapir.
• Family Lophiodontidae. Lophiodon.
• Hyracotherium.
• Family Palaeotheriidae. Palaeotherium.
• Family Equidae. Hipparion.
• Equus—horse.
• Family Rhinocerotidae. Rhinoceros.
• Elasmotherium.
• Family Titanotheriidae. Titanotherium (Brontops).
• Palaeosyops.
• Family Chalicotheriidae. Chalicotherium.
• Family Macraucheniidae. Macrauchenia.
• Division B. Subungulata.
• Suborder (1). Toxodontia.
• Family Astrapotheriidae. Astrapotherium.
• Family Nesodontidae. Nesodon.
• Family Toxodontidae. Toxodon.
• Family Typotheriidae. Typotherium.

• Suborder (2). Condylarthra.
• Family Phenacodontidae. Phenacodus.
• Suborder (3). Hyracoidea.
• Family Hyracidae. Procavia (Hyrax).
• Suborder (4). Amblypoda.
• Family Coryphodontidae. Coryphodon.
• Family Uintatheriidae. Uintatherium (Dinoceras).
• Suborder (5). Proboscidea.
• Family Dinotheriidae. Dinotherium.
• Family Elephantidae. Mastodon.
• Elephas—elephant.
• Group Tillodontia.
• Order 5. Rodentia.
• Suborder (1). Simplicidentata.
• Section Sciuromorpha.
• Family Castoridae. Castor—beaver.
• Section Myomorpha.
• Family Lophiomyidae. Lophiomys.
• Family Muridae. Hydromys.
• Acanthomys—spiny mouse.
• Mus—mouse.
• Family Spalacidae. Bathyergus.
• Family Dipodidae. Dipus—jerboa.
• Pedetes—Cape jumping-hare.
• Section Hystricomorpha.
• Family Hystricidae. Hystrix—porcupine.
• Family Chinchillidae. Chinchilla.
• Lagostomus—viscacha.

• Family Dasyproctidae. Coelogenys—paca.
• Dasyprocta—agouti.
• Family Caviidae. Cavia—guinea-pig.
• Hydrochaerus—capybara.
• Suborder (2). Duplicidentata.
• Family Leporidae. Lepus—hare and rabbit.
• Order 6. Carnivora.
• Suborder (1). Creodonta.
• Family Hyaenodontidae. Hyaenodon.
• Suborder (2). Carnivora vera or Fissipedia.
• Section Æluroidea.
• Family Felidae. Felis—cat, lion, tiger.
• Machaerodus—sabre-toothed lion.
• Family Viverridae. Viverra—civet.
• Paradoxurus—palm civet.
• Family Protelidae. Proteles—aard wolf.
• Family Hyaenidae. Hyaena.
• Section Cynoidea.
• Family Canidae. Canis—dog, wolf, fox.
• Section Arctoidea.
• Family Ursidae. Ursus—bear.
• Family Mustelidae. Latax—sea otter.
• Suborder (3). Pinnipedia.
• Family Otariidae. Otaria—sea lion.
• Family Trichechidae. Trichechus—walrus.
• Family Phocidae. Ogmorhinus—sea leopard.
• Order 7. Insectivora.
• Suborder (1). Dermoptera.
• Family Galeopithecidae. Galeopithecus—'flying lemur'.

• Suborder (2). Insectivora vera.
• Family Macroscelidae. Macroscelides—jumping shrew.
• Family Erinaceidae. Erinaceus—hedgehog.
• Gymnura.
• Family Soricidae. Sorex—shrew.
• Family Talpidae. Talpa—mole.
• Family Potamogalidae. Potamogale.
• Family Solenodontidae. Solenodon.
• Family Centetidae. Microgale.
• Centetes—tenrec.
• Family Chrysochloridae. Chrysochloris—golden mole.
• Order 8. Chiroptera.
• Suborder (1). Megachiroptera.
• Family Pteropidae. Pteropus—flying fox.
• Suborder (2). Microchiroptera.
• Family Rhinolophidae. Horseshoe bats.
• Family Phyllostomatidae. Desmodus—vampire.
• Order 9. Primates.
• Suborder (1). Lemuroidea.
• Family Tarsiidae. Tarsius—tarsier.
• Family Chiromyidae. Chiromys—aye aye.
• Suborder (2). Anthropoidea.
• Family Hapalidae. Hapale—marmoset.
• Family Cebidae. Mycetes—howling monkey.
• Ateles—spider monkey.
• Family Cercopithecidae. Cynocephalus—baboon.
• Macacus.
• Colobus.
• Family Simiidae. Hylobates—gibbon.
• Simia—orang.
• Gorilla.
• Anthropopithecus—chimpanzee.
• Family Hominidae. Homo—man.

SUBPHYLUM A. HEMICHORDATA.

The subphylum includes three genera, Balanoglossus[19], Cephalodiscus and Rhabdopleura; and perhaps a fourth, Phoronis.

The skeletal structures found in Balanoglossus[20] are all endoskeletal. They include:

(1) The notochord. This arises as a diverticulum from the alimentary canal which grows forwards into the proboscis and extends beyond the front end of the central nervous system. It is hypoblastic in origin and arises in the same way as does the notochord of Amphioxus. Its cells become highly vacuolated and take on the typical notochordal structure[21]. The cavity of the primitive diverticulum becomes obliterated in front, but behind it opens throughout life into the alimentary canal.

(2) The axial skeletal rods. These are a pair of chitinous rods which lie ventral to the notochord and in the collar region unite to form a single mass.

(3) The branchial skeleton. The gill bars separating the gill slits from one another are strengthened by chitinous rods in a way closely similar to that in Amphioxus. But between one primary forked rod and the next there are two secondary unforked rods—not one, as in Amphioxus.

(4) The chondroid tissue. This is of mesoblastic origin and may be regarded as an imperfect sheath for the notochord.

In Cephalodiscus and Rhabdopleura as in Balanoglossus the notochord forms a small diverticulum growing forwards from the alimentary canal into the proboscis stalk.

Recent researches on Phoronis[22] show the existence in the collar region of the larva (Actinotrocha) of a paired organ, which is regarded by its discoverer as representing a double notochord.

SUBPHYLUM B. UROCHORDATA (Tunicata).

Skeletal structures of epiblastic and hypoblastic origin occur in the Urochordata. Most Tunicates are invested by a thick gelatinous test which often contains calcareous spicules, and serves as a supporting organ for the soft body. The cells of this test are mesodermal in origin.

In larval Tunicata and in adults of the group Larvacea the tail is supported by a typical notochord, which is confined to the tail. In all Tunicata except Larvacea all trace of the notochord is lost in the adult.

SUBPHYLUM C. CEPHALOCHORDATA.

Fig. 3. Diagram of the skeleton of Amphioxus lanceolatus × 3
(after a drawing in the Index collection at the Brit. Mus.).

1. skeleton of dorsal fin.	5. branchial skeleton.
2. notochord.	6. septa separating the
3. neural tube.	myotomes.
4. buccal skeleton.	7. skeleton of ventral fin.

This subphylum includes the well-known genus Amphioxus[23]. In Amphioxus the skeleton is very simple. It contains no trace of cartilage or bone and remains throughout life in a condition corresponding to a very early stage in Vertebrata. The skeleton of Amphioxus is partly hypoblastic, partly mesoblastic in origin.

(a) Hypoblastic skeleton.

The notochord (fig. 3, 2) is an elastic rod extending along the whole length of the body past the anterior end of the nerve cord. It lies ventral to the nerve cord, and shows no trace of segmentation. It is chiefly made up of greatly vacuolated cells containing lymph, but near the dorsal and ventral surfaces the cells are less vacuolated. The notochord is immediately surrounded by a structureless cuticular layer, the chordal sheath, and outside this comes the mesoblastic skeletogenous layer, which also surrounds the nerve cord.

The branchial skeleton. This consists of a series of chitinous elastic rods which strengthen the gill bars and are alternately forked and unforked ventrally. The forked rods are primary, and are U-shaped in section, the unforked rods are secondary, and are circular in section. All these rods are united at intervals by transverse rods.

(b) Mesoblastic skeleton.

The buccal skeleton. On each side of the mouth there is a curved bar resembling the notochord in structure. The bars are segmented, and each segment bears a smaller rod which supports a tentacle, the whole forming the buccal skeleton (fig. 3, 4).

The notochord is enclosed in a thick sheath of connective tissue continuous with a thinner sheath round the nerve cord. The sheaths of the notochord and nerve cord together form the skeletogenous layer, and prolongations of it form the myomeres or septa between the myotomes or segments of the great lateral muscles of the body.

The skeleton of each median fin consists of small cubical masses of a gelatinous substance arranged in rows (fig. 3, 1 and 7), and serving to strengthen the fins.

The animals included in this great group all possess an internal axial skeleton forming the vertebral column or back-bone; and a dorsal spinal cord. The vertebral column is developed from the skeletogenous layer, which surrounds the spinal cord together with the notochord and its sheath; and in the great majority of cases the notochord becomes more or less modified and reduced in the adult. In some cases the notochord remains unmodified and the skeletogenous layer surrounding it is not segmented to form vertebrae, but in every case the neural arches which protect the spinal cord are segmented. The notochord never extends further forwards than the mid-brain.

All true vertebrates possess a cranium or skeletal box enclosing the brain.

(I.) Cyclostomata.

The mouth in living forms is suctorial and is not supported by jaws. In some fossil forms the character of the mouth is unknown.

Order I. Marsipobranchii[24].

In these animals limbs and limb girdles are always completely absent. They have no exoskeleton except horny teeth.

The endoskeleton, excluding the notochord, is entirely cartilaginous or membranous. The axial skeleton consists of a cartilaginous cranium without jaws, succeeded by a thick persistent notochord enveloped in a sheath. The notochord in living forms is unsegmented, but segmented cartilaginous neural arches are present in some cases. A complicated series of cartilaginous elements occurs in relation to the mouth, gills, and sense organs. The median fins are supported by cartilaginous pieces, the radiale. The order includes the Lampreys and Hags.

Order II. Ostracodermi[25].

The forms included in this group have long been extinct, being known only from beds of Upper Silurian and Lower Devonian age. They differ much from all other known animals. The exoskeleton is always greatly developed and includes (1) large bony plates covering the anterior region; (2) scales covering the posterior region. The plates are deeply marked by canals belonging to dermal sense organs. Jaws are unknown, and arches for the support of the appendicular skeleton are rudimentary or absent. The tail is heterocercal (see p. 60).

Suborder (1). Heterostraci.

The exoskeleton consists principally of calcifications forming dorsal and ventral shields which cover the head and abdominal region; the dorsal shield is formed of a few plates firmly united, the ventral shield of a single plate. The shields are composed of three layers, the middle layer being traversed by canals belonging to the dermal sense organs which open to the exterior by a series of pores. The tail is sometimes covered by scales. The orbits are widely separated and laterally placed. Paired appendages are absent. These curious forms are found in beds of Upper Silurian and Lower Devonian age. One of the best known genera is Pteraspis.

Suborder (2). Osteostraci.

The exoskeleton as in the Heterostraci consists of shields and scales, the shields being divisible into three layers. The anterior part of the body is covered dorsally by a single large shield which differs from those of the Heterostraci in having the inner layer ossified. The middle layer contains canals for the passage of blood vessels, but the exoskeleton shows no impressions of dermal sense organs. The posterior part of the body is covered by large quadrangular scales. Paired appendages are absent, but median dorsal and caudal fins occur supported by scales, not fin-rays. Cephalaspis, the best known of these animals, occurs in beds of Lower Devonian age.

Suborder (3). Antiarcha.

The exoskeleton is formed of bony plates, the dorsal and ventral shields each consisting of several symmetrically arranged pieces. The tail may be covered with small scales or may be naked. The head is articulated with the trunk, and its angles are drawn out into a pair of segmented paddle-like appendages, covered with dermal plates. The orbits are close together. A dorsal fin and traces of mouth parts occur in Pterichthys, but the endoskeleton is unknown. The best known forms Pterichthys[26] and Asterolepis occur in beds of Lower Devonian age.

General account of the skeleton of

Marsipobranchii.

The Marsipobranchii are worm-like animals. The living forms include two families, the Myxinoidei (Hags)—genera Myxine and Bdellostoma—and the Petromyzontidae (Lampreys).

Three species of Petromyzon are known, P. fluviatilis, P. marinus and P. planeri. The larval forms were for a long time thought to belong to a separate genus and were called Ammocoetes.

The Myxinoids, although very highly specialised in their own way, are at distinctly a lower stage of development than the adult Lamprey, and come nearer to the larval Lamprey or Ammocoete.

Spinal column.

Fig. 4. A, dorsal; B, lateral and C, ventral view of the
skull of Petromyzon marinus × 1 (after Parker).

1. horny teeth.	8. lateral distal mandibular.
2. labial cartilage.	9. lingual cartilage.
3. anterior dorsal cartilage.	10. branchial basket.
4. posterior dorsal cartilage.	11. cartilaginous cup supporting
5. nasal capsule.	pericardium.
6. auditory capsule.	12. sheath of notochord.
7. dorsal portion of trabeculae.	13. neural plate.

In Myxinoids and larval lampreys, the notochord is enclosed in a thick chordal sheath, in connection with which in the tail region there occur cartilaginous pieces forming neural arch elements. In the trunk region, however, no cartilage occurs in connection with the spinal column, the only cartilage present being that forming the radiale of the dorsal fin. On the other hand in most species of lamprey (Petromyzon) cartilaginous pieces forming imperfect neural arches (fig. 4, B, 13) are found lying in the tough skeletogenous layer dorsal to the notochord, and extending throughout the whole length of the trunk and tail. Two of these pieces, which are probably homologous with the neural plates (see p. 72) of Elasmobranchs, occur to each neuromere, or segment as determined by the spinal nerves. The dorsal and caudal fins are supported by paired cartilaginous radiale which are connected proximally with the skeletogenous layer.

The Skull.

In Myxinoids the cranium is a mere cartilaginous floor without side walls or roof, and the trabeculae[27] end without growing forwards into cornua. In Lampreys the trabeculae grow forwards and send up plates of cartilage which meet above (fig. 4, 7) and form side walls and a roof for part of the brain case. In Lampreys a labial suctorial apparatus is well developed, including a large ring-like piece of cartilage (fig. 4, 2) which supports the oral funnel and bears a large armament of horny teeth. In Myxinoids on the other hand the labial skeleton is small and consists merely of barbels round the mouth.

The olfactory organ of Myxinoids has a very curious skeleton. It is covered with a kind of grating of cartilage which is prolonged in front into a tube composed of a series of imperfect cartilaginous rings. In Lampreys the olfactory organ opens merely by a short membranous passage. In correlation with the small development of the labial suctorial apparatus in Myxinoids the lingual apparatus is very greatly developed. The tongue in Myxine has been said to 'dominate the whole body' (Parker). It is supported by a great median cartilaginous bar which when followed forwards first becomes bifid and still further forwards becomes four-cleft.

The horny teeth in Myxinoids are chiefly borne on the very large supralingual apparatus. They form a double series arranged in the form of an arch. In Myxine there are seven large teeth and nine small ones on each side. In Bdellostoma the teeth of the two rows are more equal in size. In Bdellostoma and Myxine it has been shown that imperfect calcified teeth occur below the horny teeth.

In Lampreys the lingual apparatus (fig. 4, C, 9) is well developed, but not excessively so. It consists of a long median cartilaginous bar which ends in front with a semicircular piece of cartilage supporting the median part of the tongue.

In both Myxinoids and Lampreys there is a complicated branchial basket apparatus, but while in Myxinoids the basket apparatus is interbranchial, formed deep within the head near the hypoblastic lining of the throat, in Lampreys it is extra-branchial and formed outside the head cavities (fig. 4, 10). The two sides of the basket apparatus in Myxine are not symmetrical. In the interbranchial basket apparatus of Myxinoids the hyoid and first and second branchial arches can be recognised. Traces of the interbranchial skeleton of Myxinoids can be detected in Lampreys, and similarly in Myxinoids, there are indications of the extra-branchial skeleton of Petromyzon. The branchial basket in Lampreys forms at its posterior end a kind of cup which supports the pericardium (fig. 4, 11).

A remarkable Cyclostome named Palaeospondylus[28] has recently been described from the Scottish Old Red Sandstone. It differs however from all living Cyclostomes, in having a spinal column formed of distinct vertebrae with well-developed neural arches. The caudal fin is well developed and the dorsal radiale are forked as in lampreys. The skull is well calcified and the auditory capsules are specially large. The mouth is very similar to that of lampreys, being circular and without jaws; it is provided with barbels or cirri. There is no trace of limbs and the average length is only about 1-1½ inches.

The mouth is supported by definite jaws.

ICHTHYOPSIDA.

The epiblastic exoskeleton is generally unimportant, the mesoblastic exoskeleton is usually well developed.

The notochord with its membranous sheath (1) may remain unmodified, or (2) may be replaced by bone or cartilage derived from the skeletogenous layer, or (3) may be calcified to a varying extent.

The first vertebra is not homologous throughout the whole series and so is not strictly comparable to the atlas of Sauropsids and Mammals.

The centra of the vertebrae have no epiphyses. The skull may be (a) incomplete and membranous, or (b) more or less cartilaginous, or (c) bony. Membrane bones are not included in the cranial walls, and there are large unossified tracts in the skull. When membrane bones are developed in connection with the skull, a large parasphenoid occurs. The basisphenoid is always small or absent. The skull may be immovably fixed to the vertebral column, or may articulate with it by a single or double occipital condyle. When the occipital condyle is double, it is formed by the exoccipitals, and the basi-occipital is small or unossified. The mandible may be (a) cartilaginous, (b) partially ossified, or (c) membrane bones may be developed in connection with it,—if so, there is usually more than one membrane bone developed in connection with each half.

There are at least four pairs of branchial arches present during development. The sternum, if present, is not costal in origin.

Class I. Pisces.

The exoskeleton is in the form of scales, which may be entirely mesoblastic or dermal in origin (e.g. cycloid and ctenoid scales), or may be formed of both mesoblast and epiblast (e.g. placoid and ganoid scales). Large bony plates may be derived from both these types of scale. In general fish with a greatly developed dermal armour have the endoskeleton poorly developed; and the converse also holds good.

The integument of the dorsal and ventral surfaces is commonly prolonged into longitudinal unpaired fins, supported by an internal skeleton. These fins are distinguished according to their position as dorsal, caudal and anal fins. The dorsal and anal fins are used chiefly as directing organs, the caudal fin is however a most important organ of propulsion.

Three types of tail are found in fishes, viz.:—

1. The diphycercal, in which the axis is straight and the tail is one-bladed and symmetrical, an equal proportion of radiale[29] being attached to the upper and lower surfaces of the axis.

2. The heterocercal, in which the tail is asymmetrical and the axis is bent upwards, the proportion of radiale or of fin-rays attached to its upper surface being much smaller than that attached to its lower surface.

3. The homocercal, in which the tail though externally symmetrical, so far resembling the diphycercal type, is internally really heterocercal, the great majority of the radiale or of the fin-rays being attached to the lower surface of the axis.

The cranium in the simplest cases (e.g. Selachii) forms a cartilaginous box enclosing the brain and sense organs; in bony fishes it is greatly complicated. When palatine or pterygoid bones are present they are formed by the ossification of cartilage; in Sauropsida and Mammalia they are laid down as membrane bones. There is no tympanic cavity or auditory ossicle in relation to the ear.

There are two principal types of suspensorium by means of which the jaws are attached to the cranium:—

(1) The Autostylic. This is the primitive condition in which the mandibular arch articulates with the base of the cranium in front of the hyoid and in a similar manner.

(2) The Hyostylic. In this case the mandibular arch becomes connected with the hyomandibular and supported by the hyoid arch. These terms are more fully discussed in Chapter VIII.

There is always an internal framework supporting the gills; it usually consists of the hyoid arch and five, rarely six or seven, pairs of branchial arches. The limbs are represented by two pairs of fins, the pectoral and the pelvic; they are not divided into proximal, middle and distal portions. The ribs do not unite with a median ventral sternum, or meet in the mid-ventral line in any other way in the trunk region.

Order I. Elasmobranchii.

The exoskeleton is in the form of placoid scales which are sometimes so numerous as to give the whole skin a rough surface forming shagreen. In some cases the placoid scales are enlarged to form plates or spines capped or coated with enamel. These spines may be imbedded in the flesh in front of the paired or unpaired fins, or may be attached to the tail. They are specially characteristic of the suborder Acanthodii. The endoskeleton is cartilaginous and true bone is never found. Much of the skeleton, especially of the vertebral column, is however often calcified, this being especially well seen in the anterior part of the vertebral column of Rays (Raiidae). In living forms cartilaginous biconcave vertebrae are always well developed, but in some extinct forms the notochord persists unconstricted. Neural and haemal arches are however always developed; they sometimes remain separate, sometimes fuse with the centra. Ribs are often wanting and when present are often not separated off from the vertebrae. The cranium is a simple cartilaginous box whose most prominent parts are the capsules which enclose the sense organs. The skull is sometimes immovably fixed to the vertebral column, sometimes articulates with it by means of two condyles. There is no operculum and no representative of the maxilla or premaxillae. The teeth are very variable. Large pectoral and pelvic fins always occur.

The Elasmobranchii may be divided into four suborders:—

(1) Ichthyotomi.

(2) Pleuropterygii.

(3) Selachii.

(4) Acanthodii.

Suborder (1). Ichthyotomi[30].

The members of this suborder range from the Devonian to the Permian and so have long been extinct.

The endoskeletal cartilage has granular calcifications evenly distributed throughout it. The notochord is unconstricted, but the neural and haemal arches are well-developed, and the neural spines are long and slender. There is a continuous dorsal fin with separate basalia and radiale. The tail is diphycercal, and the pectoral fins are typical archipterygia[31]. The pelvic fins of the male are prolonged to form claspers.

The best known of these primitive Elasmobranchs are the Pleuracanthidae.

Suborder (2). Pleuropterygii.

This suborder was formed for the reception of Cladoselache, an Elasmobranch found in the Lower Carboniferous of Ohio[32].

The exoskeleton is in the form of small, thickly-studded dermal denticles. The vertebral centra are unossified, and the tail is strongly heterocercal. There were certainly five, perhaps seven gill slits, and the suspensorium is apparently hyostylic. The paired fins are, according to the view which derives them by concentration from continuous lateral folds, the most primitive known (see p. 129) and claspers are absent.

Suborder (3). Selachii.

Cartilaginous or partially calcified biconcave vertebrae are always well developed; they constrict the notochord intervertebrally. The neural and haemal arches and spines are stout and intercalary cartilages (interdorsalia) are present. The tail is heterocercal, but in some cases (Squatina) approaches the diphycercal condition. In most cases the suspensorium is hyostylic, the jaws being attached to the cranium by means of the hyomandibular, and the palato-pterygo-quadrate bar not being fused to the cranium. There are generally five pairs of branchial arches, and gill rays are borne on the posterior surface of the hyoid arch, and on both the anterior and posterior surfaces of the first four branchial arches. The Notidanidae differ from most Selachians in two respects, first as regards the suspensorium,—Meckel's cartilage articulating directly with the palato-pterygo-quadrate bar, and not being connected with the hyoid arch; and secondly as regards the number of branchial arches,—six pairs occurring in Hexanchus and seven in Heptanchus.

The pectoral fins are without the segmented axis of the archipterygium. In most cases they are sharply marked off from the body and lie almost at right angles to it; but in the Rays they have the form of lateral expansions in the same plane as the body, from which they are not sharply marked off. The pelvic fins in the male bear long grooved cartilaginous rods which are accessory copulatory organs or claspers.

There are two principal groups of Selachii, the Squalidae or Sharks and Dogfish, and the Batoidei or Skates and Rays. The Squalidae have the shape of ordinary fish, the pectoral fins are vertically placed and the body ends in a powerful heterocercal tail. The Batoidei have flattened bodies owing to the great size and horizontal position of the pectoral fins. The tail is long and thin and is often armed with spines. The teeth in Selachii differ much in character in the different forms, and are always arranged in numerous rows. They are generally pointed and triangular or conical in the Squalidae, while in the Batoidei they are often broad and flattened.

Suborder (4). Acanthodii.

The fishes included in this group are all extinct and in some respects are intermediate between Elasmobranchii and Ganoidei. The body is elongated and closely covered with small scales consisting of dentine enamelled at the surface. The notochord is persistent and the calcification of the endoskeletal cartilage is only superficial. The tail is heterocercal. The jaws bear small conical teeth, or in some cases are toothless. The skeleton of all the fins differs from that of modern Elasmobranchs in having the cartilaginous radiale much reduced, and the fins are nearly always each provided with an anterior spine, which except in the case of the pectoral fins is merely inserted between the muscles. These spines are really enormous dermal fin-rays; the pectoral fin-spine is articulated to the pectoral girdle.

The suborder includes many well-known extinct forms like Acanthodes and Diplacanthus; it ranges from the Devonian to the Permian.

PISCES, HOLOCEPHALI.

Order II. Holocephali.

This order includes a single suborder only.

Suborder. Chimaeroidei.

Fig. 5. Skull of a male Chimaera monstrosa (after Hubrecht).

1. nasal capsule.	6. auditory capsule.
2. cartilaginous appendage to	7. interorbital septum.
the fronto-nasal region.	8. mandible articulating with
3. erectile appendage.	an outgrowth from the posterior
4. foramen by which the	part of the palato-pterygo-quadrate.
ophthalmic nerves leave the orbit.	9. teeth.
5. foramen by which the	10. labial cartilage.
ophthalmic branch of the Vth nerve	II. III. V. VII. IX. X. foramina
enters the orbit.	for the passage of cranial nerves.

These singular fish have the skin smooth and in living forms almost or quite scaleless. The palato-pterygo-quadrate bar and hyomandibular are fused to the cranium, and Meckel's cartilage articulates directly with the part corresponding to the quadrate. The skull is distinctly articulated with the spinal column, the notochord is persistent and unconstricted, and the skeletogenous layer shows no trace of metameric segmentation, though in the neural arches this segmentation is readily traceable. The neural arches of the first few vertebrae are fused together and completely surround the notochord, while they do not in other parts of the body. The tail is diphycercal. Of the living genera, in Callorhynchus there is no trace of calcification in the skeletogenous layer, while in Chimaera rings of calcification are found, there being three to five for each vertebra as indicated by the foramina for the exit of the spinal nerves. The pelvic fins are produced into claspers. Besides the living genera Chimaera, Harriotta and Callorhynchus a fair number of fossil forms are known, e.g. Ischyodus.

Order III. Ganoidei.

The fishes included under the term Ganoidei form a very heterogeneous group, some of which closely approach the Dipnoi, others the Elasmobranchii, others the Teleostei. The great majority of them are extinct, only eight living genera being known; these are all inhabitants of the northern hemisphere, and with the exception of Acipenser, which is both fluviatile and marine, are entirely confined to fresh water.

The following is a list of the living genera of Ganoids with their respective habitats:—

Acipenser. Rivers and seas of the northern hemisphere.

Scaphirhynchus. Mississippi and rivers of Central Asia.

Polyodon (Spatularia). Mississippi.

Psephurus. Yan-tse-kiang, and Hoangho.

Polypterus. Rivers of tropical Africa.

Calamoichthys. Some rivers of West Africa.

Lepidosteus. Freshwaters of Central and North America and Cuba.

Amia. Rivers of Carolina.

The exoskeleton is very variable, thus the body may be:—

(a) Naked or with minute stellate ossifications as in the Polyodontidae. (b) Partially covered with large detached bony plates as in Scaphirhynchus and Acipenser. (c) Entirely covered with rhomboidal ganoid scales as in Lepidosteus, Polypterus, Palaeoniscus and many extinct forms. (d) Covered with rounded scales shaped like the cycloid scales of Teleosteans as in Amia. (e) Having the trunk and part of the tail covered with rhomboidal scales, and the remainder of the tail with rounded scales as in Trissolepis.

The teeth also are very variable. The endoskeleton shows every stage of transition from an almost entirely cartilaginous state as in Acipenser to a purely bony state as in Lepidosteus. Sometimes, as in Acipenser, the notochord persists, and its sheath is unsegmented; sometimes, as in Lepidosteus, there are fully formed vertebrae. The tail may be heterocercal, as in Acipenser, or diphycercal as in Polypterus. The cartilaginous cranium is always covered with external membrane bone to a greater or less extent, and the suspensorium is markedly hyostylic. The pectoral girdle is formed of two parts, one endoskeletal and cartilaginous, corresponding with the pectoral girdle of Elasmobranchs, and one exoskeletal and formed of membrane bones, corresponding with the clavicular bones of Teleosteans. The pelvic fins are always abdominal. The fins often, as in Polypterus, have spines (fulcra) attached to their anterior borders.

The order Ganoidei may be divided into three suborders:

(1) Chondrostei. Living genera Acipenser, Scaphirhynchus, Polyodon and Psephurus.

(2) Crossopterygii. Living genera Polypterus and Calamoichthys.

(3) Holostei. Living genera Lepidosteus and Amia.

Suborder (1). Chondrostei.

The skull is immovably fixed to the vertebral column. By far the greater part of the skeleton is cartilaginous. The notochord is persistent and unconstricted, its sheath is membranous, but cartilaginous neural and haemal arches are developed. Intercalary pieces (interdorsalia) occur between the neural arches, and similar pieces (interventralia) between the haemal arches. The cranium is covered with membrane bone, and teeth are but slightly developed. The tail is heterocercal. Gill rays occur on the hyoid arch, and the gills are protected by a bony operculum attached to the hyomandibular. The skin (1) may be almost or quite naked, (2) may carry bony plates arranged in rows, or may be covered (3) with rhomboidal scales, or (4) partly with rhomboidal, partly with cycloidal scales.

Suborder (2). Crossopterygii.

The exoskeleton has the form of cycloidal or rhomboidal scales. The condition of the vertebral column differs in the different genera. Sometimes, as in Polypterus, there are well-developed ossified vertebrae; sometimes, as in many extinct forms, the notochord persists and is unconstricted. The tail may be diphycercal or heterocercal. The pectoral and sometimes the pelvic fins consist of an endoskeletal axis bearing a fringe of dermal rays.

Suborder (3). Holostei.

The exoskeleton has the form of cycloidal or rhomboidal scales. The notochord is constricted and its sheath is segmented and ossified, forming distinct vertebrae, which are generally biconcave, sometimes opisthocoelous (Lepidosteus). The cartilaginous cranium is largely replaced by bone, and in connection with it we find not only membrane bone, but cartilage bone, as the basi-occipital, exoccipitals, and pro-otic are ossified. The tail is heterocercal. The suspensorium resembles that of Teleosteans, consisting of a proximal ossification, the hyomandibular, which is movably articulated to the skull and a distal ossification, the symplectic. The two are separated by some unossified cartilage. The cartilaginous upper and lower jaws are to a great extent surrounded and replaced by a series of membrane bones.

Order IV. Teleostei.

The exoskeleton is sometimes absent but generally consists of overlapping cycloid or ctenoid scales. Bony plates are sometimes present, as in the Siluridae, or the body may be encased in a complete armour of calcified plates, as in Ostracion. Enamel is however never present, and the plates are entirely mesodermal. The skeleton is bony, but in the skull much cartilage generally remains. The vertebral centra are usually deeply biconcave, and the tail is of the masked heterocercal type distinguished as homocercal. In the skull the occipital region is always completely ossified, while the sphenoidal region is generally less ossified. The skull has usually a very large number of membrane bones developed in connection with it. The teeth vary much in character in the different members of the order, but are as a rule numerous and pointed, and are ankylosed to the bone. The suspensorium is hyostylic and the jaws have much the same arrangement as in the Holostei. There are five pairs of branchial arches, of which all except the last bear gill rays. A series of dermal opercular bones is developed in connection with these arches. The pectoral girdle consists almost entirely of dermal clavicular bones. The pelvic girdle has disappeared, its place being taken by the enlarged and ossified dermal fin-rays of the pelvic fins.

The group includes the vast majority of living fish (see p. 33).

Order V. Dipnoi.

The exoskeleton is of two types; dermal bones are largely developed in the head region, while the tail and posterior part of the body may be naked or may be covered with overlapping scales. The cranium remains chiefly cartilaginous, the palato-pterygo-quadrate bar is fused with the cranium, and the suspensorium is autostylic. The gill clefts are feebly developed and open into a cavity covered by an operculum. The notochord is persistent and unconstricted, and the limbs are archipterygia. The pelvic fins are without claspers.

Suborder (1). Sirenoidei[33].

The head has well developed membrane bones. The trunk is covered with overlapping scales and bears no bony plates. Three pairs of teeth are present, two in the upper and one in the lower jaw, the two principal pairs of teeth are borne on the palato-pterygoids and splenials, while the third pair are found in the vomerine region. The tail is diphycercal in living forms. In the extinct Dipteridae it is heterocercal. The pectoral girdle includes both membrane and cartilage bones. The pelvic girdle consists of a single bilaterally symmetrical piece of cartilage.

This suborder is represented by the living genera Ceratodus, Protopterus and Lepidosiren, and among extinct forms by the Dipteridae and others.

Suborder (2). Arthrodira.

Bony plates are developed not only on the head but also on the anterior part of the trunk, where they consist of a dorsal, a ventral, and a pair of lateral plates which articulate with the cranial shield. The posterior part of the trunk is naked. The tail is diphycercal. The jaws are shear-like, and their margins are usually provided with pointed teeth whose bases fuse with the tissue of the jaw and constitute dental plates. There seem to have been three pairs of these plates, arranged as in the Sirenoidei, the principal ones in the upper jaw being borne on the palato-pterygoids. Small pelvic fins are present, but pectoral fins are unknown.

The Arthrodira occur chiefly in beds of Devonian and Carboniferous age. Two of the best known genera are Coccosteus from the European Devonian and Dinichthys, a large predatory form from the lower Carboniferous of Ohio.

Scyllium canicula.

I. EXOSKELETON.

The exoskeleton of the dogfish is mainly composed of placoid scales, each of which consists of a little bony base imbedded in the skin, bearing a small backwardly-directed spine formed of dentine capped with enamel. The scales are larger on the dorsal than on the ventral surface, and on the jaws they are specially large and regularly arranged in rows, there forming the teeth. The margins of the jaws or lips are without scales.

A second exoskeletal structure is found in the fins, all of which, both paired and unpaired, have, in addition to their cartilaginous endoskeleton, large numbers of long slender horny fibres, the fin-rays, which are of exoskeletal origin.

II. ENDOSKELETON.

The endoskeleton of the dogfish consists almost entirely of cartilage, which however may become calcified in places, e.g. the centrum of each vertebra is lined by a layer of calcified tissue.

The endoskeleton is divisible into an axial portion consisting of the vertebral column, skull, and skeleton of the median fins, and an appendicular portion consisting of the skeleton of the paired fins and their girdles.

1. The Axial Skeleton.

A. The Vertebral Column and Ribs.

The vertebral column consists of a series of some hundred and thirty vertebrae, each of which is united with its predecessor and successor in such a way as to allow a large amount of flexibility.

These vertebrae are developed round an unsegmented rod, the notochord, which forms the axial support of the embryo. The notochord remains continuous throughout the whole vertebral column, but is greatly constricted opposite the middle of each vertebra, and thus rendered moniliform. The vertebrae are divided into two groups, an anterior group of trunk vertebrae, and a posterior group of caudal or tail vertebrae.

A typical vertebra consists of a middle portion, the centrum, a dorsal portion, the dorsal or neural arch, which surrounds the spinal cord, and a ventral portion, the ventral or haemal arch, which similarly encloses a space.

The tail vertebrae of the dogfish have this typical arrangement, the trunk vertebrae have the haemal arches modified.

Each centrum is a short cylinder of cartilage surrounding an hourglass-shaped cavity occupied by the notochord. The neural arches are composed of three separate elements, the vertebral neural plates (basidorsalia), intervertebral neural plates (interdorsalia), and neural spines (supradorsalia).

The vertebral neural plates are in the adult fused with their respective centra, and are notched behind for the exit of the ventral (motor) roots of the spinal nerves. The intervertebral neural plates are polygonal pieces alternating with the vertebral neural plates; they are notched behind, but at a more dorsal level than are the vertebral neural plates, for the exit of the dorsal or sensory roots of the spinal nerves.

The neural spines are small patches of cartilage filling up the gaps between the dorsal ends of the neural plates.

The haemal arches (basiventralia) differ much in the trunk and tail portions of the vertebral column. In the trunk portion the centra are flattened below, and the two halves of the haemal arch diverge from one another as blunt ventri-lateral processes to which short cartilaginous rods, the ribs, are attached. Further back at about vertebra 37, the two halves of the haemal arch project downwards and meet forming a complete arch. Further back still, towards the hind end of the tail, the haemal arches bear median haemal spines (ventrispinalia).

B. The Skull.

The skull of the dogfish remains cartilaginous throughout the life of the animal, and has consequently a far more simple structure than have the skulls of higher animals, in which complication has been produced by the development of bone.

The skull consists of the following parts:—

(1) a dorsal portion, the cranium, which lodges the brain, and to the sides of which the capsules of the auditory and olfactory sense organs are united. The cranium may be compared to an unsegmented continuation of the vertebral column;

(2) a number of ventral structures, disconnected or only loosely connected with the cranium. These together constitute the visceral skeleton forming the jaws and supporting the gills.

(1) The Cranium.

The Cranium is an oblong box, with a flattened floor and a more irregular roof. Its sides are expanded in front owing to the olfactory capsules, and behind owing to the auditory capsules, while in the middle they are deeply hollowed to form the orbits.

(a) On the dorsal surface of the cranium the following points should be noticed. First at the anterior end, the large thin-walled nasal or olfactory capsules (fig. 6, 1), each of which is drawn out into a narrow cartilaginous process.

The olfactory capsules have no ventral walls, and are separated from one another by the internasal septum, which is drawn out into a third slender process. These three processes together constitute the rostrum (fig. 6, 2).

Behind the olfactory capsules comes a large, nearly circular, hole, the anterior fontanelle, slightly behind which are the two ophthalmic foramina. The dorsal and ventral boundaries of the orbits are respectively formed by the prominent supra-orbital and suborbital ridges. Behind are the auditory capsules (fig. 6, 8), each of which is marked by a pair of prominent ridges, converging towards the middle line to a pair of apertures. These apertures communicate with two canals, the aqueductus vestibuli, which lead into the internal ear. The two ridges lodge respectively the anterior and posterior vertical semicircular canals of the ear.

(b) The principal structures to be noted in a side view of the cranium are contained in the orbit or eye-cavity. Near the base of the orbit at its anterior end is seen the small orbitonasal foramen (fig. 6, 7), for the passage of blood-vessels, not nerves. Above it is the large ophthalmic foramen (fig. 6, 5) so prominent in a dorsal view of the skull; through it the ophthalmic branches of the fifth and seventh nerves pass. Slightly further back near the ventral surface is the large optic foramen (fig. 6, II.) for the passage of the second nerve. Vertically above the optic foramen, near the dorsal surface, is the very small foramen for the fourth nerve (fig. 6, IV.). Behind and a little above the optic foramen is another small aperture, the foramen for the third nerve. Behind and slightly below this is the large foramen for the sixth and main branches of the fifth and seventh nerves (fig. 6, V.). In front of and slightly below this foramen are seen two other small apertures; the more anterior and ventral of these (fig. 6, 4) is for the passage of a vessel connecting the efferent artery of the hyoid gill with the internal carotid artery inside the skull, the more posterior and dorsal is for the interorbital canal (fig. 6, 3) which unites the two orbital sinuses. Above and very slightly in front of the large foramen for the sixth and main parts of the fifth and seventh nerves, are two small foramina (fig. 6, Va., and VIIa.), through which the ophthalmic branches of the fifth and seventh nerves enter the orbit. Behind and slightly below the large foramen just mentioned is a small hole through which the external carotid enters the orbit (fig. 6, 9).

Fig. 6. Lateral view of the skull of a Dogfish (Scyllium
canicula) × 2/3.

1. nasal capsule.	10. ethmo-palatine ligament.
2. rostrum.	11. palato-pterygo-quadrate bar.
3. interorbital canal.	12. Meckel's cartilage.
4. foramen for hyoidean artery.	13. hyomandibular.
5. foramen for the exit of the	14. cerato-hyal.
ophthalmic branches of	15. pharyngo-branchial.
Vth and VIIth nerves.	16. epi-branchial.
6. foramen through which the	17. cerato-branchial.
external carotid leaves the	18. gill filaments, nearly all have
orbit.	been cut off short for the
7. orbitonasal foramen.	sake of clearness.
8. auditory capsule.	19. extra-branchial
9. foramen through which the	20. pre-spiracular ligament.
external carotid enters the	II. III. IV. V. Va. VIIa. foramina
orbit.	for passage of cranial nerves.

Behind the orbit is the auditory capsule. This is marked below by a prominent surface for the articulation of the hyomandibular, above which is the deep postorbital groove for the passage of a blood-vessel, connecting the orbital and anterior cardinal sinuses.

(c) Passing to the posterior end of the cranium: in the centre is seen the large foramen magnum through which the brain and spinal cord communicate. The notochord enters the skull just below this foramen, and on each side of the notochord is a projection, the occipital condyle, by which the first vertebra articulates with the skull.

External to the condyles are the prominent pneumogastric foramina for the passage of the tenth nerves, and further to the sides, just beyond the posterior vertical semicircular canals, are a pair of deep pits in which lie the foramina for the ninth nerves (fig. 6, IX).

(d) The broad and flat ventral surface of the cranium is continued in front as the internasal septum and terminated laterally by the suborbital ridges. At a little behind the middle it is traversed by two shallow grooves along which the internal carotid arteries run. At the divergent ends of these grooves are seen two small apertures through which the external carotids enter the orbit (fig. 6, 9), and at the point where they meet is a single small aperture through which the internal carotid enters the cranium.

(2) The Visceral Skeleton.

The Visceral skeleton forms a series of seven cartilaginous arches or hoops, surrounding the anterior part of the alimentary canal, and enclosing a wide but rather shallow space.

(a) The first or mandibular arch is the largest of the series, and forms the upper and lower jaws. Each half of the upper jaw or palato-pterygo-quadrate bar is formed by a thick cartilaginous rod which meets its fellow in the middle line in front, the two being united by ligament. Each half is connected to the cranium just in front of the orbit by the ethmo-palatine ligament (fig. 6, 10), and at its hind end articulates with one of the halves of the lower jaw. Each half of the lower jaw or Meckel's cartilage (fig. 6, 12) is a cartilaginous bar, wide behind but narrow in front, where it is united to its fellow by a median ligament. Imbedded in the tissue external to the upper jaw are a pair of labial cartilages, and a similar but smaller pair are imbedded in the tissue external to the lower jaw.

The jaws are developed from a structure whose dorsal and ventral portions subsequently become of very different importance. The ventral portion forms both upper and lower jaws, the former being developed as an outgrowth from the latter. The dorsal portion forms only the pre-spiracular ligament (fig. 6, 20), a strong fibrous band containing a nodule of cartilage, and running from the anterior part of the auditory capsule to the point where the jaws are connected with the hyomandibular.

(b) The hyoid arch consists of a pair of cartilaginous rods which are attached at their dorsal ends to the cranium, and are united ventrally by a broad median plate of cartilage, the basi-hyal. Each rod is divided into a dorsal portion, the hyomandibular and a ventral portion, the cerato-hyal. The hyomandibular (fig. 6, 13) is a short stout rod of cartilage projecting outwards, and somewhat backwards and downwards from the cranium, with which it articulates behind the orbit and below the postorbital groove. Its distal end articulates with a rather long slender bar, the cerato-hyal (fig. 6, 14), which is in its turn attached to the side of the basi-hyal. The basi-hyal is a broad plate, rounded in front and drawn out behind into two processes to which the two halves of the first branchial arch are attached. The posterior surfaces of both hyomandibular and cerato-hyal bear slender cartilaginous processes, the gill rays. The hyoid arch forms the main suspensorium or means by which the jaws are attached to the cranium. This attachment is chiefly brought about by a series of short ligaments which connect the posterior ends of both upper and lower jaws with the hyomandibular, but there is also a ligament connecting the lower jaw with the cerato-hyal. The attachment of the jaws to the cranium is also partially effected by the pre-spiracular and ethmo-palatine ligaments.

(c) Each of the five branchial arches is a hoop, incomplete above and formed of four or more pieces of cartilage. The most dorsal elements, the pharyngo-branchials, are flattened, pointed plates whose free inner ends run obliquely backwards, and terminate below the vertebral column. They are connected at their outer ends with the short broad epi-branchials (fig. 6, 16) which lie at the sides of the pharynx. From the epi-branchials arise the long cerato-branchials (fig. 6, 17) which run forwards and inwards along the ventral wall of the pharynx. The first four cerato-branchials are connected with small rods, the hypo-branchials, which run backwards to meet one another in the middle line. The last two pairs of hypo-branchials and the fifth cerato-branchials are connected with a broad median plate, the basibranchial. Along the outer sides of the second, third and fourth cerato-branchials are found elongated curved rods, the extra-branchials (fig. 6, 19). The epi-branchials and cerato-branchials bear gill rays along their posterior borders.

C. The Skeleton of the Median Fins.

The dorsal fins have a skeleton consisting of a series of short cartilaginous rods, the basals or basalia, which slope obliquely backwards. Their bases are imbedded in the muscles of the back, while their free ends bear a number of small polygonal cartilaginous plates, the radials or radiale. Associated with this cartilaginous skeleton are a number of long slender horny fibres, the fin-rays, which have been already referred to in connection with the exoskeleton. The skeleton of the other median fins mainly consists of these fibres, the cartilaginous portion being reduced or absent.

2. The Appendicular Skeleton.

This includes the skeleton of the two pairs of limbs and of their respective girdles.

The Pectoral girdle forms a crescent-shaped hoop of cartilage, incomplete above and lying just behind the visceral skeleton. The mid-ventral part of the hoop is the thinnest portion, and is drawn out in front into a short rounded process which is cupped dorsally and supports part of the floor of the pericardium (fig. 7, 1). On each side of this flattened mid-ventral portion the arch becomes very thick and bears on its outer border a surface with which the three basal cartilages of the fin articulate. The dorsal ends or scapular portions of the girdle form a pair of gradually tapering horns.

The Pectoral fin articulates with the pectoral girdle by means of three basalia or basal cartilages, the propterygium, meso-pterygium and meta-pterygium. The most anterior and the smallest of these is the propterygium (fig. 7, 5), while the most posterior one, the meta-pterygium (fig. 7, 3), is much the largest. Along the outer borders of the three basalia are arranged a series of close set cartilaginous pieces, the radiale. The propterygium supports only a single radial, which is however much larger than any of the others. The meso-pterygium also supports only a single radial which divides distally.

Fig. 7. Semidorsal view of the pectoral girdle and fins
of a Dogfish (Scyllium canicula) × 2/3.
The gaps between the radiale are blackened.

1. hollow in the mid-ventral part	5. propterygium.
of the pectoral girdle which	6. propterygial radial.
supports the pericardium.	7. meso-pterygial radial.
2. dorsal (scapular portion) of	8. meta-pterygial radial.
pectoral girdle.	9. outline of the distal part of
3. meta-pterygium.	the fin which is supported
4. meso-pterygium.	by horny fin-rays.

The meta-pterygium bears about twelve long narrow radials, the first nine of which are traversed by a transverse joint at about two-thirds of the way from their origin. Succeeding the radials are a series of small polygonal pieces of cartilage arranged in one or more rows and attached to the ends of the radials, and finally the fin is completed by the dermal fin-rays.

Fig. 8. Dorsal view of the pelvic girdle and fins of a
male Dogfish (Scyllium canicula).

1. pelvic girdle.	3. clasper.
2. basi-pterygium.	4. radiale.

The Pelvic Girdle is much smaller than the pectoral. It is formed of a stout nearly straight bar of cartilage placed transversely across the ventral region of the body. The bar has no dorsal or lateral extensions, and is terminated by short blunt processes. It bears on its posterior surface a pair of facets with which the pelvic fins articulate.

The Pelvic Fin is smaller and more simply constructed than is the pectoral. It consists of a long, somewhat curved rod, the basi-pterygium (fig. 8, 2), running directly backwards on the inner side of the fin, and articulating in front with the pelvic girdle. From its outer side arise a series of about fourteen parallel cartilaginous radials which bear smaller polygonal pieces. The anterior one or two of these radials may articulate independently with the pelvic girdle. In the adult male dogfish the distal end of the basi-pterygium bears a stout rod nearly as long as itself, and grooved on the dorsal surface. This is the skeleton of the clasper (fig. 8, 3).

I. EXOSKELETON.

The exoskeleton includes

(1) Scales. These are of the type known as cycloid and consist of flat rounded plates composed of concentrically arranged laminae of calcified matter, with the posterior margin entire. The anterior end of each scale is imbedded in the skin and is overlapped by the preceding scales.

(2) The teeth. These are small, pointed, calcified structures arranged in large groups on the premaxillae, mandible, vomer, and superior and inferior pharyngeal bones.

(3) The fin-rays. These are delicate, nearly straight bony rods which support the fins.

II. ENDOSKELETON.

The endoskeleton of the Codfish, though partially cartilaginous, is mainly ossified.

It is divisible into an axial portion, including the skull, vertebral column, ribs, and skeleton of the median fins, and an appendicular portion, including the skeleton of the paired fins and their girdles.

1. The Axial Skeleton.

A. The Vertebral Column.

This consists of a series of some fifty-two vertebrae, all completely ossified.

It is divisible into two regions only, viz. the trunk region, the vertebrae of which bear movable ribs, and the caudal or tail region, the vertebrae of which do not bear movable ribs.

Trunk vertebrae.

These are seventeen in number; the ninth may be described as typical of them all. It consists of a short deeply biconcave centrum whose two cavities communicate by a narrow central canal. From the dorsal surface of the anterior half of the centrum arise two strong plates, the dorsal or neural processes, which are directed obliquely backwards and meet forming the dorsal or neural arch. This is produced into a long backwardly-directed dorsal or neural spine.

From the lower part of the anterior edge of each neural arch arise a pair of blunt triangular projections which overhang the posterior half of the preceding centrum, and bear a pair of flattened surfaces which correspond to the anterior or prezygapophyses of most vertebrae, they differ however from ordinary prezygapophyses in the fact that they look downwards and outwards. From the posterior end of the centrum arise a pair of short blunt processes each of which bears an upwardly- and inwardly-directed articulating surface corresponding to a postzygapophysis.

The two halves of the ventral arch form a pair of large ventri-lateral processes which arise from the anterior half of the centrum and pass outwards and slightly backwards and downwards.

Behind these there arises on each vertebra a second outgrowth which is small and flattened, and like the ventri-lateral process serves to protect the air-bladder. The surface of the centrum is marked by more or less wedge-shaped depressions, one in the mid-dorsal line, and two on the ventral surface immediately mesiad to the bases of the ventri-lateral process. There are also a number of smaller depressions.

The space between one centrum and the next is in the fresh skeleton filled up by the gelatinous remains of the notochord.

The first few vertebrae differ from the others in having very short centra and no ventri-lateral processes.

The first vertebra comes into very close relation to the posterior part of the skull, articulating with the exoccipitals. In the next few vertebrae the centra gradually lengthen, and at the fourth or fifth vertebra the ventri-lateral processes appear and gradually increase in size as followed back. They likewise gradually come to arise at a lower level on the centrum, and also become more and more downwardly directed, till at the last trunk vertebra they nearly meet.

The neural spines of the anterior trunk vertebrae are much longer than those of the posterior ones, that of the first vertebra being the largest and longest of all, and articulating with the skull. The spinal nerves pass out through wide notches or spaces between the successive neural arches.

Caudal vertebrae.

The caudal vertebrae are about thirty-five in number, each consists of a centrum with a slender backwardly-directed dorsal or neural arch, similar to those of the posterior trunk vertebrae. The two halves of the ventral or haemal arch however do not form outwardly-directed ventri-lateral processes, but arise on the ventral surface of the centrum, and passing downwards meet and enclose a space; they thus form a complete canal, and are prolonged into a backwardly-directed ventral or haemal spine. The anterior haemal arches are much larger than the corresponding neural arches, but when followed back they gradually decrease in size, till at about the twenty-fourth caudal vertebra they are nearly as small as the neural arches. The last caudal vertebra is succeeded by a much flattened hypural bone or urostyle, which together with the posterior neural and haemal spines supports the tail-fin.

B. The Ribs.

The ribs are slender, more or less cylindrical bones attached to the poster-dorsal faces of the ventri-lateral processes of all the trunk vertebrae except the first and second. The earlier ones are thicker and more curved; the later ones thinner and more nearly straight. The ribs are homologous with the distal parts of the haemal arches of the caudal vertebrae.

Associated with the ribs are a second series of rib-like bones, the intermuscular bones. These are slender, curved bones which arise from the ribs or from the ventri-lateral processes at a distance of about an inch from the centra, and curve upwards, outwards and backwards. In the anterior region where the ventri-lateral processes are short they arise from the ribs, further back they arise from the ventri-lateral processes.

C. The Unpaired or Median Fins.

These are six in number, three being dorsal, one caudal and two anal.

The dorsal and anal fins each consist of two sets of structures, the fin-rays and the interspinous bones. Each fin-ray forms a delicate, nearly straight, bony rod which becomes thickened and bifurcated at its proximal or vertebral end, while distally it is transversely jointed and flexible, frequently also becoming more or less flattened.

The first dorsal fin has thirteen rays, the second, sixteen to nineteen, the third, seventeen to nineteen. The first anal fin has about twenty-two, the second anal fourteen. In each fin the posterior rays rapidly decrease in size when followed back.

The interspinous bones of the dorsal and anal fins alternate with the neural and haemal spines respectively, and form short, forwardly-projecting bones, each attached proximally to the base of the corresponding fin-ray.

The caudal fin consists of a series of about forty-three rays which radiate from the posterior end of the vertebral column, being connected with the urostyle or hypural bone, and with the posterior neural and haemal spines without the intervention of interspinous bones. Like the other fin-rays those forming the caudal fin are transversely jointed, and are widened and frayed out distally.

The tail-fin in the Cod is homocercal, i.e. it appears to be symmetrically developed round the posterior end of the vertebral column, though in reality a much greater proportion is attached below the end of the vertebral column than above it. It is a masked heterocercal tail.

The Skull.

Owing to the fact that very little cartilage remains in the skull of the adult Codfish, its relation to the completely cartilaginous skull of the Dogfish is not easily seen. Before describing it therefore, the skull of the Salmon will be described, as it forms an intermediate type.

THE SKULL OF THE SALMON[36].

The Salmon's skull consists of (1) the chondrocranium, which remains partly cartilaginous and is partly converted into cartilage bone, especially in the occipital region, (2) a large series of plate-like membrane bones.

The Chondrocranium.

This is an elongated structure, wide behind owing to the fusion of the large auditory capsules with the cranium, and elongated and tapering considerably in front; in the middle it is much contracted by the large orbital cavities.

Dorsal surface of the Cranium.

In the centre of the posterior end of the dorsal surface is the supra-occipital (fig. 9, A, 1) with a prominent posterior ridge. It is separated by two tracts of unossified cartilage from the large series of bones connected with the auditory organ. The first of these is the epi-otic (fig. 9, 2), which is separated by only a narrow tract of cartilage from the supra-occipital, and is continuous laterally with the large pterotic (fig. 9, A, 3) which overlaps in front a smaller bone, the sphenotic (fig. 9, 4). Both epi-otic and pterotic are drawn out into rather prominent backwardly-projecting processes.

Fig. 9. A. dorsal and B. ventral view of the cranium of a Salmon
(Salmo salar) from which most of the membrane bones have been
removed (after Parker). Cartilage is dotted.

1. supra-occipital.	12. opisthotic.
2. epi-otic.	13. alisphenoid.
3. pterotic.	14. orbitosphenoid.
4. sphenotic.	16. foramen for passage of an
5. frontal.	artery.
6. median ethmoid.	17. pro-otic.
7. parietal.	18. articular surface for
8. lateral ethmoid.	hyomandibular.
9. parasphenoid.	II. VII. IX. X. foramina for the
10. vomer.	passage of cranial nerves.
11. exoccipital.

The greater part of the remainder of the dorsal surface is formed of unossified cartilage which is pierced by three large vacuities or fontanelles. The anterior fontanelle is unpaired, and lies far forward near the anterior end of the long cartilaginous snout, the two larger posterior ones lie just in front of the supra-occipital and lead into the cranial cavity. In front of the orbit the skull widens again, and is marked by two considerable lateral ethmoid (fig. 9, 8) ossifications. In front of these are a pair of deep pits, the nasal fossae, at the base of which are a pair of foramina through which the olfactory nerves pass out; they communicate with a space, the middle narial cavity, seen in a longitudinal section of the skull.

The long cartilaginous snout is more or less bifid in front, especially in the male (fig. 9).

Posterior end of the Cranium.

The foramen magnum forms a large round hole leading into the cranial cavity, and is bounded laterally by the two exoccipitals and below by them, and to a very slight extent by the basi-occipital, the three bones together forming a concave occipital condyle by which the vertebral column articulates with the skull.

The exoccipitals are connected laterally with a fourth pair of auditory bones, the opisthotics, and just meet the epi-otics dorsolaterally, while dorsally they are separated by a wide tract of unossified cartilage from the supra-occipital.

The opisthotics are connected laterally with the pterotics.

Side of the Cranium.

At the posterior end is seen the basi-occipital in contact above with the exoccipital, which is pierced by a prominent foramen for the exit of the tenth nerve. In front of this lies a small foramen, sometimes double, for the ninth nerve.

Fig. 10. Lateral view of the chondrocranium of a Salmon
(Salmo salar) (after Parker). A few membrane bones are also shown.
Cartilage is dotted.

1. supra-occipital.	12. olfactory pit; the vomerine
2. epi-otic.	teeth are seen just below.
3. pterotic.	14. pro-otic.
4. opisthotic.	15. basisphenoid.
5. exoccipital.	16. foramen for the passage of
6. basi-occipital.	an artery.
7. parasphenoid.	17. anterior fontanelle.
8. sphenotic.	18. posterior fontanelle.
9. alisphenoid.	I. II. V. VII. IX. X. foramina
10. orbitosphenoid.	for the passage of cranial nerves.
11. lateral- or ectethmoid.

In front of the exoccipital is the large pro-otic pierced by two prominent foramina. Through the more dorsal of these (fig. 10, VII.) the facial nerve passes out, while the more ventral (fig. 10, 16) is for the passage of an artery. Dorsal to the exoccipital are the opisthotic and pterotic, and dorsal to the pro-otic is the sphenotic. The pterotic is marked by a prominent groove often lined by cartilage, which is continued forwards along a tract of cartilage between the pro-otic and sphenotic. With this groove the hyomandibular articulates.

There are considerable ossifications in the sphenoidal region of the side of the cranium. The anterior boundary of the posterior fontanelle is formed by the large alisphenoid, which is continuous behind with the pro-otic and sphenotic, and below with a slender basisphenoid. Both in front of and behind the basisphenoid there are considerable vacuities in the walls of the cranium; through the posterior of these openings (fig. 10, V.) the main part of the trigeminal nerve passes out, and through the anterior one, the optic (fig. 10, II.). The alisphenoid is continuous in front with the orbitosphenoid (fig. 10, 10), which is pierced by the foramen for the exit of the first nerve (fig. 10, I.), and in front of the orbitosphenoid there is a large vacuity. The lateral ethmoid is seen in the side view as well as in the dorsal view. Further forwards are seen the olfactory pits, and the long cartilaginous snout.

A ventral view of the cartilaginous cranium shows much the same points as the side view. The basisphenoid appears on the surface immediately in front of the basi-occipital.

The Skull with membrane bones.

The dorsal surface. The greater part of the dorsal surface in front of the supra-occipital is overlaid by a pair of large rough frontals (figs. 9, A, 5, and 10, 5). They cover the posterior fontanelles and stretch over from the sphenotic to the lateral ethmoid, forming a roof for the orbit. They meet in the middle line behind, but in front are separated by a narrow tract of unossified cartilage, and are overlapped by the median ethmoid (figs. 9, A, 6, and 11, 6). At the sides of the supra-occipital behind the frontals are a pair of small parietals (figs. 9, A, 7, and 11, 7).

Fig. 11. Lateral view of the skull of a Salmon (Salmo salar)
(after Parker). Cartilage is dotted.

1. supra-occipital.	19. maxillae
2. epi-otic.	20. premaxillae.
3. pterotic.	21. articular.
4. sphenotic.	22. angular.
5. frontal.	23. dentary.
6. median ethmoid.	24. hyomandibular.
7. parietal.	25. symplectic.
8. nasal.	26. epi-hyal.
9. lachrymal.	27. cerato-hyal.
10. suborbital.	28. hypo-hyal.
11. supra-orbital.	29. glosso-hyal.
12. cartilaginous sclerotic.	30. opercular.
13. ossification in sclerotic.	31. sub-opercular.
14. meso-pterygoid.	32. infra-opercular.
15. meta-pterygoid.	33. pre-opercular.
16. palatine.	34. supratemporal
17. jugal.	35. branchiostegal rays.
18. quadrate.	36. basi-branchiostegal.

In a ventral view the cranium is seen to be chiefly covered by two large membrane bones, the parasphenoid (fig. 9, B, 9) behind, the vomer in front. A view of the posterior end differs from that of the cartilaginous cranium only in the fact that the end of the parasphenoid appears lying ventral to the basi-occipital.

The lateral view differs very markedly from that of the cartilaginous cranium, there being a great development of membrane bone in connection with the jaws and branchial apparatus. Lying dorsally are seen the median ethmoid, frontal, parietal, and supra-occipital as before. Lying external to the middle of the median ethmoid is seen the small nasal (fig. 11, 8), and below the hinder part is the lachrymal. The lachrymal (fig. 11, 9) forms the first of a series of seven small bones which surround the orbit forming the orbital ring. Of these the one lying immediately in the mid-ventral line of the orbit is the suborbital, while the one lying in the mid-dorsal line and attached to the frontal is the supra-orbital (fig. 11, 11). The orbit has a cartilaginous sclerotic in which are two small ossifications (fig. 11, 13) laterally placed.

Bones of the upper jaw.

The palato-pterygo-quadrate bar is in a very different condition from that of the dogfish, it is partially cartilaginous, partially converted into cartilage bone, partially overlapped by membrane bone. It is narrow in front but becomes much broader and deeper when followed back. Its anterior end forms the palatine which bears teeth, and in front is completely ossified, while behind the cartilage is only sheathed by bone.

Just behind the palatine the outer part of the cartilage is ossified, forming two small bones, the pterygoid and meso-pterygoid, while behind them is a larger, somewhat square bone, the meta-pterygoid (fig. 11, 15).

Below the meta-pterygoid is a tract of unossified cartilage, and then comes the quadrate (fig. 11, 18).

The lower angle of the quadrate bears a cartilaginous condyle with which the mandible articulates. In front of the palatine the cartilaginous snout is overlapped by three membrane bones, the jugal, maxilla and premaxillae.

The premaxillae (fig. 11, 20), the largest of these, overlaps the maxilla behind; both bones bear teeth. The jugal (fig. 11, 17) lies above the maxilla and overlaps it in front.

The lower jaw.

The lower jaw is a strong bar and is like the upper jaw, partly cartilaginous, forming Meckel's cartilage, partly ossified, and sheathed to a considerable extent in membrane bone.

The outer side and posterior end is ossified, forming the large articular (fig. 11, 21), but the condyle is cartilaginous and the anterior part of the articular forms merely a splint on the outer side of Meckel's cartilage, which extends beyond it for a considerable distance. The angle of the jaw just below the condyle is formed by a small angular (fig. 11, 22), and the anterior two-thirds of the jaw is sheathed in the large tooth-bearing dentary (fig. 11, 23).

The Hyoid arch.

The hyoid arch has a number of ossifications in it and is closely connected with the mandibular arch.

The hyomandibular (fig. 11, 24) is a large bone which articulates with a shallow groove lined by cartilage and formed partly in the pterotic, partly in front of it. The hyomandibular is overlapped in front by the meta-pterygoid, while below it tapers and is succeeded by a small area of unossified cartilage followed by the forwardly-directed symplectic which fits into a groove in the quadrate.

The unossified tract between the hyomandibular and symplectic is continuous in front with a strong bar, which remains partly cartilaginous and is partly converted into cartilage bone. The proximal part is ossified, forming the epi-hyal, the middle part forms the cerato-hyal (fig. 11, 27), in front of which is the small hypo-hyal. The hyoid arches of the two sides are united by the large tooth-bearing glosso-hyal (fig. 11, 29). Attached to the lower surface of the hyoid arch are a series of twelve flat branchiostegal rays (fig. 11, 35). Each overlaps the one in front of it, the posterior one being the largest. The branchiostegal rays of the two sides are united in front by an unpaired membrane bone, the basi-branchiostegal (fig. 11, 36).

Opercular bones. Behind the hyomandibular there is a large bony plate, the operculum, formed of four large membrane bones. The anterior of these, the pre-opercular (fig. 11, 33), is crescentic in shape, and with its upper end a small supratemporal (fig. 11, 34) is connected.

Behind the upper part of the pre-opercular is the largest of the opercular bones, the opercular proper. Its lower edge overlaps the sub-opercular, and both opercular and sub-opercular are overlapped by the infra-opercular (fig. 11, 32) in front. The infra-opercular is in its turn overlapped by the pre-opercular.

Branchial arches.

There are five branchial arches, the first four of which bear gill rays. Each of the first three consists of a shorter upper portion directed obliquely backwards and outwards, and a longer lower portion forming a right angle with the upper and directed obliquely forwards and inwards. The greater part of each arch is ossified.

The upper part of either of the first two consists of a short tapering pharyngo-branchial directed inwards, and of a long epi-branchial tipped with cartilage at both ends. The junction of the upper and lower parts is formed by a cartilaginous hinge-joint between the epi-branchial and cerato-branchial. The cerato-branchial is a long bony rod separated by a short area of cartilage from the hypo-branchial, which is succeeded by the basibranchial meeting its fellow in the middle line. The fourth arch has a short epi-branchial and no ossified pharyngo-branchial, while the fifth is reduced to little more than the cerato-branchial, which bears a few teeth on its inner edge. All the branchial arches have projecting from their surfaces a number of little processes which act as strainers. The first and fourth arches have one series of these, the second and third have two.

THE SKULL OF THE CODFISH[37].

A full description having been already given of the Salmon's skull, that of the Codfish will be described in a briefer manner. The skull is very fully ossified, and the great number of plate-like bones render it a very complicated structure.

The Cranium.

At the posterior end of the dorsal surface is the large supra-occipital, which is drawn out behind into the large blade-like occipital spine. On each side of the supra-occipital are the small irregular parietals, while in front of it the roof of the skull is mainly formed by the very large unpaired frontal.

A complicated series of bones are developed in connection with the auditory capsule, which forms a large projecting mass united with the side of the cranium and drawn out behind into a pair of strong processes, the epi-otic and parotic processes. Both these processes are connected behind with a large V-shaped bone, the post-temporal (fig. 13, 1), which will be described when dealing with the pectoral girdle. The epi-otic process is formed by the epi-otic, which is continuous in front with the parietal. The parotic process is formed by two larger bones, a more dorsal one, the pterotic, and a more ventral and internal one, the opisthotic, which is continuous in front with the large pro-otic. Intervening between the pterotic and frontal is another rather large bone, the sphenotic, this articulates below with the pro-otic. The pterotic and sphenotic together give rise to a large concave surface by which the hyomandibular articulates with the cranium. Several of the cranial nerves pass out through the bones of the auditory capsule. The ninth leaves by a foramen near the posterior border of the opisthotic, the fifth and seventh by a notch in the anterior border of the pro-otic.

A number of bones are likewise developed in connection with the orbit forming the orbital ring. Of these the most anterior, the lachrymal, is much the largest, the others are five to seven in number, the most ventral being the suborbital. The sclerotic coat of the eye is cartilaginous.

Two pairs of bones and one unpaired bone are developed in connection with the olfactory capsules, of these, the nasals are narrow bones lying next the lachrymals, but nearer the middle line; they overlap the second pair of bones, the irregular lateral ethmoids. These meet one another in the middle line, and are overlapped behind by the frontal. They articulate laterally with the lachrymal and palatine, and ventrally with the parasphenoid.

In a posterior view the foramen magnum and the four bones which surround it and together form the occipital segment are well seen. On the ventral side is the basi-occipital, terminated posteriorly by a slightly concave surface which articulates with the centrum of the first vertebra. The sides of the foramen magnum are formed by the exoccipitals, a pair of very irregular bones, pierced by a pair of prominent foramina for the exit of the tenth nerves. The exoccipitals also bear a pair of surfaces for articulation with corresponding ones on the neural arch of the first vertebra. The most dorsal of the four bones is the supra-occipital.

On the ventral surface of the cranium in front of the basi-occipital is seen the parasphenoid, a very long narrow bone which underlies the greater part of the cranium. Behind, it articulates dorsally with the basi-occipital and dorsolaterally with the pro-otics and opisthotics, in front it articulates dorsally with the lateral ethmoid and ventrally with the vomer. At the sides of the parasphenoid are the small alisphenoids articulating above with the postfrontals, in front with the frontals, and behind with the pro-otics.

The vomer is an unpaired bone lying immediately in front of the parasphenoid. In front it terminates with a thickened curved margin bearing several rows of small teeth; behind it tapers out into a long process which underlies the anterior part of the parasphenoid. Immediately dorsal to the vomer is another median bone, the median ethmoid; this is truncated in front and tapers out behind into a process which fits into a groove on the ventral side of the frontal.

Bones in connection with the upper jaw.

These bear a close resemblance to those of the Salmon. The most anterior bone is the premaxillae, a thick curved bone meeting its fellow in the middle line. The point of junction of the two is drawn out into a short process, and the oral surface is thickly covered with small teeth. The dorsal ends of the premaxillae are seen in the fresh skull to meet a large patch of cartilage. Behind the premaxillae is the maxilla, a long rod-like toothless bone, somewhat expanded at the upper end where it articulates with the premaxillae and vomer.

Articulating in front with the anterior end of the maxilla and with the lateral ethmoid is a very irregular bone, the palatine (fig. 12, 1); it articulates behind with two flat bones, the pterygoid and meso-pterygoid. The pterygoid is united behind with two more bones, the quadrate (fig. 12, 4) and meta-pterygoid. The quadrate is a rather stout irregular bone, bearing on its lower surface a prominent saddle-shaped articulating surface for the mandible. The palatine, pterygoid and quadrate bones are the ossified representatives of the palato-pterygo-quadrate bar of the Dogfish.

Fig. 12. Mandibular and hyoid arches of a Cod (Gadus
morrhua) × ½ (Brit. Mus.).

1. palatine.	9. articular.
2. meso-pterygoid.	10. dentary.
3. pterygoid.	11. inter-hyal.
4. quadrate.	12. epi-hyal.
5. symplectic.	13. cerato-hyal.
6. meta-pterygoid.	14. hypo-hyal.
7. hyomandibular.	15. uro-hyal.
8. angular.	16. branchiostegal rays.

The quadrate is united behind with the symplectic (fig. 12, 5), and the meta-pterygoid with the symplectic and hyomandibular, both of which bones will be described immediately in connection with the hyoid arch.

The Lower jaw.

The lower jaw or mandible like that of the Salmon is partly cartilaginous, forming Meckel's cartilage, partly formed of cartilage bone, partly of membrane bone. Meckel's cartilage is of course not seen in the dried skull.

The lower jaw includes one cartilage bone, the articular (fig. 12, 9), this is a large bone connected by a saddle-shaped surface with the quadrate. Meckel's cartilage lies in a groove on its under surface, and projects beyond it in front. The angular is a small thick bone united to the lower surface of the articular at its posterior end. The dentary (fig. 12, 10) is a large tooth-bearing bone meeting its fellow in the middle line in front, while the articular fits into a deep notch at its posterior end.

The hyoid arch.

The hyomandibular (fig. 12, 7) is a large irregular bone, articulating by a prominent rounded head with the sphenotic and pterotic. It is united in front with the meta-pterygoid and symplectic, and sends off behind a strong process which articulates with the opercular. The symplectic is a long somewhat triangular bone drawn out in front into a process which fits into a groove on the inner surface of the quadrate. The distal portion of the hyoid arch is strongly developed and consists of first the inter-hyal (fig. 12, 11), a short bony rod, which articulates dorsally with a patch of cartilage intervening between the posterior part of the hyomandibular and the symplectic. Below it is united with the apex of the triangular epi-hyal, a bone suturally connected with the large cerato-hyal (fig. 12, 13) which unites distally with two small hypo-hyals. To the cerato-hyal are attached a series of seven strong curved cylindrical rods, the branchiostegal rays. The first of these is the smallest and they increase in size up to the last. The four dorsal ones are attached to the outer surface of the cerato-hyal, the three ventral ones to its inner surface. Interposed between the hypo-hyals of the two sides is an unpaired somewhat triangular plate, the uro-hyal or basi-branchiostegal (fig. 12, 15).

The branchial arches.

The branchial arches are five in number and consist of the following parts on each side. The dorsal end is formed of the supra-pharyngeal bone, a large irregular bone covered ventrally with teeth of a fair size, and representing the fused pharyngo-branchials of the four anterior arches. Its external surface is continuous with four small epi-branchials which pass horizontally backwards and outwards. Their distal ends meet four long cerato-branchials which are directed forwards and inwards and form the principal part of the arches.

Each of the first three cerato-branchials articulates ventrally with a hypo-branchial, and the hypo-branchials of the two sides are united in the middle line by an unpaired basibranchial. The third hypo-branchial is much flattened. The fourth cerato-branchial is united by cartilage with the posterior surface of the third hypo-branchial, which it meets near the middle line.

The fifth arch consists only of the cerato-branchial, a wide structure covered with teeth and generally called the inferior pharyngeal bone.

The skeleton of the operculum consists of the same four bones as in the Salmon, namely the opercular, the infra-opercular, the pre-opercular and the sub-opercular. Of these the anterior bone, the pre-opercular, is the largest, while the infra-opercular is the smallest. The opercular has a facet for articulation with the hyomandibular.

2. The Appendicular Skeleton.

The Pectoral girdle.

This is of a highly specialised type. Membrane bones are greatly developed, and the cartilage bones, the scapula and coracoid, are much reduced in size and importance.

Fig. 13. The right half of the pectoral girdle and right
pectoral fin of a Cod (Gadus morrhua) × ½ (Brit. Mus.).

1. post-temporal.	5. scapula.
2. supra-clavicle.	6. post-clavicle.
3. clavicle.	7. brachial ossicles.
4. coracoid.	8. dermal fin-rays.

The largest bone in the shoulder girdle is the clavicle (fig. 13, 3), which is irregularly crescent shaped, thick in front and tapering off behind. To the outer side of its upper part is attached a thick cylindrical bone, the supra-clavicle, which passes upwards and is connected with a strong V shaped bone, the post-temporal. The apex of the V meets the supra-clavicle, the inner limb articulates with the epi-otic process, the outer with the parotic process. Projecting downwards from the upper part of the clavicle is a long bony rod, flattened proximally and cylindrical and pointed distally, this is the post-clavicle (fig. 13, 6).

The scapula (fig. 13, 5) is a small irregular plate of bone attached to the inner side of the middle of the clavicle. The coracoid[38] is a larger plate of similar character, irregularly triangular in shape, attached to the inner side of the clavicle immediately below the scapula. The scapula and coracoid bear the pectoral fin.

The Pectoral fins.

Each of these consists of four small irregular bones, the brachial ossicles (fig. 13, 7), bearing a series of about nineteen dermal fin-rays. The brachial ossicles represent the reduced and modified radiale and basalia of cartilaginous fish such as the dogfish. The fin-rays (fig. 13, 8) which form the whole external portion of the fin are long slender rods having essentially the same character as those of the unpaired fins.

The Pelvic girdle.

The pelvic girdle in the Cod as in other Teleosteans is entirely absent, its place being taken by the enlarged basi-pterygia of the fins.

The Pelvic fins.

These have a very anomalous position in the Cod, being attached to the throat in front of the pectoral girdle. Each consists of a basal portion, the basi-pterygium, and of a number of dermal rays. The basi-pterygium consists of an expanded ventral portion which meets its fellow below in the middle line, and to which the rays are attached, and of an inwardly-directed dorsal portion which also meets its fellow and is imbedded in the flesh. The rays are six in number and are long slender structures similar to those of the other fins.

EXOSKELETON.

The most primitive type of exoskeleton is that found in Elasmobranchs and formed of placoid scales; these are tooth-like structures consisting of dentine and bone capped with enamel, and have been already described (p. 4). In most Elasmobranchs they are small and their distribution is fairly uniform, but in the Thornback skate, Raia clavata, they have the form of larger, more scattered spines. In adult Holocephali and in Polyodon and Torpedo there is no exoskeleton, in young Holocephali, however, there are a few small dorsal ossifications.

The plates or scales of many Ganoids may have been formed by the gradual fusion of elements similar to these placoid scales, and often bear a number of little tooth-like processes. In Lepidosteus, Polypterus, and many extinct species, these ganoid scales, which are rhomboidal in form and united to one another by a peg and socket articulation, enclose the body in a complete armour. In Trissolepis part of the tail is covered by rhomboidal scales, while rounded scales cover the trunk and remainder of the tail. Acipenser and Scaphirhynchus have large dermal bony plates which are not rhomboidal in shape and do not cover the whole body. In Acipenser a single row extends along the middle of the back and two along each side.

The majority of Teleosteans have thin flattened scales which differ from those of Ganoids in being entirely mesodermal in origin, containing no enamel. There are two principal types of Teleostean scales, the cycloid and ctenoid. A cycloid scale is a flat thin scale with concentric markings and an entire posterior margin. A ctenoid scale differs in having its posterior margin pectinate. The Dipnoi have overlapping cycloid scales. The rounded scales of Amia and of many fossil ganoids such as Holoptychius are shaped like cycloid scales, but differ from them in being more or less coated with enamel. In Eels and some other Teleosteans the scales are completely degenerate and have almost disappeared. Some Teleosteans, like Diodon hystrix, have scales with triradiate roots from which arise long sharp spines directed backwards. These scales, which resemble teeth, contain no enamel; they become erect when the fish inflates its body into a globular form. Many Siluroids have dermal armour in the form of large bony plates which are confined to the anterior part of the body. In Ostracion the whole body is covered by hexagonal plates, closely united together.

The fin-rays are structures of dermal origin which entirely or partially support the unpaired fins, and assist the bony or cartilaginous endoskeleton in the support of the paired fins.

In Elasmobranchs, Dipnoi, and Chondrosteous ganoids the skeletons of the fins are, as a rule, about half of exoskeletal, half of endoskeletal origin, the proximal and inner portion being cartilaginous and endoskeletal, the distal and outer portion being exoskeletal, and consisting of horny or of more or less calcified fin-rays. In bony Ganoids and Teleosteans the endoskeletal parts are greatly reduced and the fins come to consist mainly of the fin-rays, which are ossified and frequently become flattened at their distal ends.

The fin-rays of the ventral part of the caudal fin are carried by the haemal arches; those of the dorsal and anal fins and of the dorsal part of the caudal fin generally by interspinous bones, which in adult Teleosteans alternate with the neural and haemal spines. In Dipnoi these interspinous bones articulate with the neural and haemal spines. In many Siluroids the anterior rays of the dorsal and pectoral fins are developed into large spines which often articulate with the endoskeleton, or are sometimes fused with the dermal armour plates. Similar spines may occur in Ganoids in front of both the dorsal and anal fins. Polypterus has a small spine or fulcrum in front of each segment of the dorsal fin. Such spines are often found fossilised, and are known as ichthyodorulites.

Similar spines are found in many Elasmobranchs, but they are simply inserted in the flesh, not articulated to the endoskeleton. They also differ from the spines of Teleosteans and Ganoids in the fact that they are covered with enamel, and often have their edges serrated like teeth. In the extinct Acanthodii they generally occur in front of all the fins, paired and unpaired.

In Trygon, the Sting-ray, the tail bears a serrated spine which is used for purposes of offence and defence. Many ichthyodorulites may have been spines of this nature fixed to the tail, rather than spines situated in front of the fins. The spines, which are always found in front of the dorsal fin in Holocephali, agree with those of Elasmobranchs in containing enamel, and with those of Teleosteans in being articulated to the endoskeleton.

Teeth.

The teeth of fish[40] are subject to a very large amount of variation, perhaps to more variation than are those of any other class of animals. Sometimes, as in adult Sturgeons, they are entirely absent, sometimes they are found on all the bones of the mouth, and also on the hyoid and branchial arches. The teeth are all originally developed in the mucous membrane of the mouth, but they afterwards generally become attached to firmer structures, especially to the jaws. In Elasmobranchs, however, they are generally simply imbedded in the tough fibrous integument of the mouth. Their attachment to the jaws may take place in three different ways.

Fig. 14. Diagram of a section through the jaw of a Shark (Odontaspis
americanus) showing the succession of teeth (Brit. Mus. from
specimen and diagram).

1. teeth in use.	cartilage.
2. teeth in reserve.	6. connective tissue.
3. skin.	7. mucous membrane of the
4. cartilage of the jaw.	mouth.
5. encrusting calcification of

(1) By an elastic hinge-joint, as in the Angler (Lophius), and the Pike (Esox lucius). In the Angler the tooth is held by a fibrous band attaching its posterior end to the subjacent bone, in the Pike by uncalcified elastic rods in the pulp cavity.

(2) By ankylosis, i.e. by the complete union of the calcified tooth substance with the subjacent bone. This is the commonest method among fish.

(3) By implantation in sockets. This method is not very common among fish. The teeth are sometimes, as in Lepidosteus, ankylosed to the base of the socket. In this genus there is along each ramus of the mandible a median row of large teeth placed in perfect sockets, and two irregular lateral rows of small teeth ankylosed to the jaw.

Dentine, enamel and cement are all represented in the teeth of fishes, but the enamel is generally very thin, and cement is but rarely developed. Dentine forms the main bulk of the teeth; it is sometimes of the normal type, but generally differs from that in higher vertebrates in being vascular, and is known as vasodentine. A third type occurs, known as osteodentine; it is traversed by canals occupied by marrow, and is closely allied to bone.

Fig. 15. Part of the lower jaw of a Shark (Galeus)
(from Owen after André).

1. teeth in use.	a Sting-ray (Trygon) which has
2. reserve teeth folded back.	pierced the jaw and affected the
3. part of the caudal spine of	growth of the teeth.

The teeth are generally continually renewed throughout life, but sometimes one set persists.

The teeth of Selachii are fundamentally identical with placoid scales. They are developed from a layer of dental germs which occurs all over the surface of the skin, except in the region of the lips. At this point the layer of tooth-producing germs extends back into the mouth, being projected by a fold of the mucous membrane (fig. 14, 7). Here new teeth are successively formed, and as they grow each is gradually brought into a position to take the place of its predecessor by the shifting outwards of the gum over the jaw. Owing to this arrangement sharks have practically an unlimited supply of teeth (figs. 14 and 15).

Two principal types of teeth are found in Elasmobranchs. In Sharks and Dogfish, on the one hand, the teeth are very numerous, simple, and sharp-pointed, and are with or without serrations and lateral cusps. Many Rays and fossil Elasmobranchs, on the other hand, have broad flattened teeth adapted for crushing shells. Intermediate conditions occur between these two extremes. Thus in Cestracion and many extinct sharks, such as Acrodus, while the median teeth are sharp, the lateral teeth are more or less flattened and adapted for crushing. In various species belonging to the genus Raia the teeth of the male are sharp, while those of the female are blunt. A very specialised dentition is met with in the Eagle-rays (Myliobatidae), in which the jaws are armed with flattened angular tooth-plates, arranged in seven rows, forming a compact pavement; the plates of the middle row are very wide and rectangular, those of the other rows are much smaller and hexagonal. Lastly, in Cochliodus the individual crushing teeth are fused, forming two pairs of spirally-coiled dental plates on each side of each jaw. Pristis, the Saw-fish, has a long flat cartilaginous snout, bearing a double row of persistently-growing teeth planted in sockets along its sides. Each tooth consists of a number of parallel dentinal columns, united at the base, but elsewhere distinct.

In the Holocephali—Chimaera, Hariotta and Callorhynchus—only three pairs of teeth or dental plates occur, two pairs in the upper jaw, one in the lower. These structures persist throughout life and grow continuously. The upper tooth structures are attached respectively to the ethmoid or vomerine region of the skull, and to the palato-pterygoids. The vomerine teeth are small, while those attached to the mandible and the palato-pterygoid region are large and bear several roughened ridges adapted for grinding food. The teeth of the two opposite sides of the jaw meet in a median symphysis. The teeth of Chimaera are more adapted for cutting, those of Callorhynchus for crushing. Many extinct forms are known, some of whose teeth are intermediate in structure between those of Chimaera and Callorhynchus.

The teeth of Ganoids are also extremely variable. Among living forms, the Holostei are more richly provided with teeth than are any other fishes, as they may occur on the premaxillae, maxillae, palatines, pterygoids, parasphenoid, vomers, dentaries, and splenials. Among the Chondrostei, on the other hand, the adult Acipenseridae are toothless; small teeth however occur in the larval sturgeon, and in Polyodon many small teeth are found attached merely to the mucous membrane of the jaws. Many fossil Ganoids have numerous flattened or knob-like teeth, borne on the maxillae, palatines, vomers and dentaries. Others have a distinctly heterodont dentition. Thus in Lepidotus the premaxillae bear chisel-like teeth, while knob-like teeth occur on the maxillae, palatines and vomers. In Rhizodus all the teeth are pointed, but while the majority are small a few very large ones are interspersed.

In Teleosteans, too, the teeth are eminently variable both in form and mode of arrangement. They may be simple and isolated, or compound, and may be borne on almost any of the bones bounding the mouth cavity, and also as in the Pike, on the hyoid and branchial arches. The splenial however never bears teeth and the pterygoid and parasphenoid only rarely, thus differing from the arrangement in the Holostei.

The isolated teeth are generally conical in form and are ankylosed to the bone that bears them. Such teeth are, with a few exceptions such as Balistes, not imbedded in sockets nor replaced vertically.

In some fish beak-like structures occur, formed partly of teeth, partly of the underlying jaw bones. These beaks are of two kinds: (1) In Scarus, the parrot fish, the premaxillae and dentaries bear numerous small, separately developed teeth, which are closely packed together and attached by their proximal ends to the bone, while their distal ends form a mosaic. Not only the teeth but the jaws which bear them are gradually worn away at the margins, while both grow continuously along their attached edge. (2) In Gymnodonts, e.g. Diodon, the beaks are formed by the coalescence of broad calcified horizontal plates, which when young are free and separated from one another by a considerable interval.

In some Teleosteans the differentiation of the teeth into biting teeth and crushing teeth is as complete as in Lepidosteus. Thus in the Wrasse (Labrus), the jaws bear conical slightly recurved teeth arranged in one or two rows, with some of the anterior ones much larger than the rest. The bones of the palate are toothless, while both upper and lower pharyngeal bones are paved with knob-like crushing teeth; such pharyngeal teeth occur also in the Carp but are attached only to the lower pharyngeal bone, the jaw bones proper being toothless.

In Dipnoi the arrangement of the teeth is very similar to that in Holocephali. The mandible bears a single pair of grinding teeth attached to the splenials, and a corresponding pair occur on the palato-pterygoids. In front of these there are a pair of small conical vomerine teeth loosely attached to the ethmoid cartilage. The palato-pterygoid teeth of Ceratodus are roughly semicircular in shape with a smooth convex inner border, and an outer border bearing a number of strongly marked ridges. The teeth of the extinct Dipteridae resemble those of Ceratodus but are more complicated.

ENDOSKELETON.

Spinal column[41].

The spinal column of fishes is divisible into only two regions, a caudal region in which the haemal arches or ribs meet one another ventrally, and a precaudal region in which they do not meet.

The various modifications of the spinal column in fishes can be best understood by comparing them with the arrangement in the simplest type known, namely Amphioxus. In Amphioxus the notochord is immediately surrounded by a structureless cuticular layer, the chordal sheath. Outside this is the skeletogenous layer, which in addition to surrounding the notochord and chordal sheath embraces the nerve cord dorsally, and laterally sends out septa forming the myomeres.

The Cartilaginous ganoids[42] Acipenser, Polyodon and Scaphirhynchus are the simplest fishes as regards their spinal column. The notochord remains permanently unconstricted and is enclosed in a chordal sheath, external to which is the skeletogenous layer. In this layer the development of cartilaginous elements has taken place. In connection with each neuromere, or segment as determined by the points of exit of the spinal nerves, there are developed two pairs of ventral cartilages, the ventral arches (basiventralia) and intercalary pieces (interventralia); and at least two pairs of dorsal pieces, the neural arches (basidorsalia) and intercalary pieces (interdorsalia). The lateral parts of the skeletogenous layer do not become converted into cartilage, so there are no traces of vertebral centra. The ventral or haemal arches meet one another ventrally and send out processes to protect the ventral vessels. The neural arches do not meet, but are united by a longitudinal elastic band.

In Cartilaginous ganoids the only indications of metameric segmentation are found in the neural and haemal arches. The case is somewhat similar with the Holocephali and Dipnoi.

In the Holocephali the notochord grows persistently throughout life, and is of uniform diameter throughout the whole body except in the cervical region and in the gradually tapering tail. The chordal sheath is very thick and includes a well-marked zone of calcification which separates an outer zone of hyaline cartilage from an inner zone. There are also a number of cartilaginous pieces derived from the skeletogenous layer which are arranged in two series, a dorsal series forming the neural arches and a ventral series forming the haemal arches. These do not, except in the cervical region, meet one another laterally round the notochord and form centra. To each neuromere there occur a pair of basidorsals, a pair of interdorsals, and one or two supradorsals. In the tail the arrangement is irregular.

In the Dipnoi as in the Holocephali the notochord grows persistently and uniformly, and the chordal sheath is thick and cartilaginous though there are no metamerically arranged centra. The neural and haemal arches and spines are cartilaginous and interbasalia (intercalary pieces) are present. The basidorsalia and basiventralia do not in Ceratodus meet round the notochord and enclose it except in the anterior part of the cervical and posterior part of the caudal region.

In Elasmobranchii the chordal sheath is weak and the skeletogenous layer strong. Biconcave cartilaginous vertebrae are developed, and as is the case in most fishes, constrict the notochord vertebrally.

Two distinct types of vertebral column can be distinguished in Elasmobranchs[43]:

1. In many extinct forms and in the living Notidanidae, Cestracion, and Squatina, the dorsal and ventral arches do not meet one another laterally round the centrum, and consequently readily come away from it.

2. In most living Elasmobranchs the arches meet laterally round the centrum.

The vertebrae are never ossified but endochondral calcification nearly always takes place, though it very rarely reaches the outer surface of the vertebrae. Elasmobranchs are sometimes subdivided into three groups according to the method in which this calcification takes place:

1. Cyclospondyli (Scymnus, Acanthias), in which the calcified matter is deposited as one ring in each vertebra.

2. Tectospondyli (Squatina, Raia, Trygon), in which there are several concentric rings of calcification.

3. Asterospondyli (Notidanidae, Scyllium, Cestracion), in which the calcified material instead of forming one simple ring, extends out in a more or less star-shaped manner.

In Heptanchus the length of the vertebral centra in the middle of the trunk is double that in the anterior and posterior portions, and as the length of the arches does not vary, the long centra carry more of them than do the short centra.

In many Rays the skull articulates with the vertebral column by distinct occipital condyles.

In Bony Ganoids the skeletogenous layer becomes calcified ectochondrally in such a way that the notochord is pinched in at intervals, and distinct vertebrae are produced. Ossification of the calcified cartilage rapidly follows. In Amia the vertebrae are biconcave, in Lepidosteus they are opisthocoelous, cup and ball joints being developed between the vertebrae in a manner unique among fishes. The notochord entirely disappears in the adult Lepidosteus, but at one stage in larval life it is expanded vertebrally and constricted intervertebrally in the manner usual in the higher vertebrata, but unknown elsewhere among fishes.

The tail of Amia is remarkable from the fact that as a rule to each neuromere, as determined by the exit of the spinal nerves, there are two centra, a posterior one which bears nothing, and an anterior one which bears the neural and haemal arches, these being throughout the vertebral column connected with the centra by cartilaginous discs.

In most Teleosteans but not in the Plectognathi the neural arches are continuous with the centra, which are nearly always deeply biconcave.

In some cases many of the anterior vertebrae are ankylosed together and to the skull. The vertebrae often articulate with one another by means of obliquely placed flattened surfaces, the zygapophyses. The centrum in early stages of development is partially cartilaginous, but the neural arches and spines in the trunk at any rate, pass directly from the membranous to the osseous condition.

Fins.

The most primitive fins are undoubtedly the unpaired ones, which probably originally arose as ridges or folds of skin along the mid-dorsal line of the body, and passed thence round the posterior end on to the ventral surface, partially corresponding in position and function to the keel of a ship.

In long 'fish' which pass through the water with an undulating motion such simple continuous fins may be the only ones found, as in Myxine. To support these median fins skeletal structures came to be developed; these show two very distinct forms, viz. cartilaginous endoskeletal pieces, the radiale, and horny exoskeletal fibres, the fin-rays. Mechanical reasons caused the fin to become concentrated at certain points and reduced at intervening regions. Thus a terminal caudal fin arose and became the chief organ of propulsion, and the dorsal and ventral fins became specialised to act as balancing organs.

In some of the earlier Elasmobranchs, the Pleuracanthidae, the endoskeletal cartilaginous radiale are directly continuous with outgrowths from the dorsal and ventral arches of the vertebrae, and form the main part of the fin. In later types of Elasmobranchs the horny exoskeletal fin-rays have comparatively greater prominence. In bony fish, as has been already stated, the horny fibres are replaced by bony rays of dermal origin, and at the same time complete reduction and disappearance of the cartilaginous radiale takes place.

The Caudal fin.

The caudal region of the spinal column in fishes is of special importance. It is distinctly marked off from the rest of the spinal column by the fact that the ventral or haemal arches meet one another and are commonly prolonged into spines, while in the trunk region they do not meet but commonly diverge from one another.

In some fish the terminal part of the caudal region of the spinal column retains the same direction as the rest of the spinal column. The blade of the caudal fin is then divided into two nearly equal portions, and is said to be diphycercal. This condition is generally regarded as the most primitive one; it occurs in the Ichthyotomi, Holocephali, all living Dipnoi, Polypterus and some extinct Crossopterygii, and a few Selachii and Teleostei. It occurs also in deep-sea fish belonging to almost every group, and under these conditions obviously cannot be regarded as primitive, but must be looked on as a feature induced by the peculiar conditions of life.

In the great majority of fish the terminal part of the caudal region of the spinal column is bent dorsalwards, and the part of the blade of the caudal fin which arises on the dorsal surface is much smaller than is that arising on the ventral surface. Such a fin is said to be heterocercal.

Strictly speaking all fish whose tails are not diphycercal have heterocercal tails, but the term is commonly applied to two-bladed tails in which the spinal column forms a definite axis running through the dorsal blade, while the ventral blade is enlarged and generally forms the functional part of the tail. Such heterocercal tails are found in nearly all Elasmobranchii, together with the living cartilaginous Ganoidei, and many extinct forms belonging to the same order; Lepidosteus, Amia, and the Dipteridae among Dipnoi, have tails which, though obviously heterocercal, are not two-bladed.

The vast majority of the Teleostei and some extinct Ganoidei have heterocercal tails of the modified type to which the term homocercal is applied. The hypural bones which support the lower half of the tail fin become much enlarged, and frequently unite to form a wedge-shaped bone which becomes ankylosed to the last ossified vertebral centrum. The fin-rays then become arranged in such a way as to produce a secondary appearance of symmetry. Some homocercal fish such as the Perch have the end of the notochord protected by a calcified or completely ossified sheath, the urostyle, to which several neural and haemal arches may be attached, and which becomes united with the centrum of the last vertebra; in others such as the Salmon the end of the notochord is protected only by laterally placed bony plates.

The Skull.

It is often impossible to draw a hard and fast line between the cranium and the vertebral column. This is the case for instance in Acipenser (fig. 18, 16) among Chondrostei, in Amia among Holostei, and in Ceratodus and Protopterus among Dipnoi. The occipital region of the skull in Amia is clearly formed of three cervical vertebrae whose centra have become absorbed into the cranium, while the neural arches and spines are still distinguishable.

Fig. 16. A. Skull of Notidanus × ½ (Brit. Mus.). B. Skull of
Cestracion × 1/3 (after Gegenbaur). In neither case are the
branchial arches shown.

1. rostrum.	6. Meckel's cartilage.
2. olfactory capsule.	7. teeth.
3. ethmo-palatine process.	8. labial cartilage.
4. palatine portion of	9. hyomandibular.
palato-pterygo-quadrate bar.	10. postorbital process.
5. quadrate portion of bar.	II. optic foramen.

The simplest type of cranium is that found in Elasmobranchs: it consists of a simple cartilaginous box, which is generally immovably fixed to the vertebral column, though in some forms, like Scymnus and Galeus, a joint is indicated, and in others, such as the Rays, one is fairly well developed. The cranium in Elasmobranchs is never bony, though the cartilage is sometimes calcified. It is drawn out laterally into an antorbital process in front of the eye, and a postorbital process behind it. The nasal capsules are always cartilaginous, and the eye, as a general rule, has a cartilaginous sclerotic investment. The cranium is often prolonged in front into a rostrum which is enormously developed in Pristis and some Rays. The cartilaginous roof of the cranium is rendered incomplete by the presence of a large hole, the anterior fontanelle.

Two pairs of labial cartilages (fig. 16, B, 8) are often present. They lie imbedded in the cheeks outside the anterior region of the jaws, and are specially large in Squatina.

As regards the visceral arches[44] the simplest and most primitive condition of the jaws is that of the Notidanidae, in which the mandibular and hyoid arches are entirely separate. In these primitive fishes the palato-pterygo-quadrate bar articulates with the postorbital process (fig. 16, 10), while further forwards it is united to the cranium by the ethmo-palatine ligament. The hyoid arch is small and is broadly overlapped by the mandibular arch. The term autostylic is used to describe this condition of the suspensorium. From this condition we pass in the one direction to that of Cestracion (fig. 16, B), in which the whole of the palato-pterygo-quadrate bar has become bound to the cranium, and in the other to that of Scyllium. In Scyllium (fig. 6), while the ethmo-palatine ligament is retained, the postorbital articulation of the palato-pterygo-quadrate has been given up, so that the palato-pterygo-quadrate comes to abut on the hyomandibular and is attached to it by ligaments. The pre-spiracular ligament (fig. 16, 20) running from the auditory capsule also assists in supporting the jaws.

Lastly we come to the purely hyostylic condition met with in Rays, in which the mandibular arch is entirely supported by the hyomandibular. In some Rays the hyoid is attached to the posterior face of the hyomandibular near its proximal end, and may even come to articulate with the cranium.

The visceral arches of Elasmobranchs may be summarised as follows:—

1. The mandibular arch, consisting of a much reduced dorsal portion, the pre-spiracular ligament, and a greatly developed ventral portion from which both upper and lower jaws are derived. The mandible (Meckel's cartilage) is the original lower member of the mandibular arch, and from it arises an outgrowth which forms the upper jaw or palato-pterygo-quadrate bar. In Scymnus this bears a few branchiostegal rays.

2. The hyoid arch, which consists of the hyomandibular and the hyoid, and bears branchiostegal rays on its posterior face.

Fig. 17. Dorsal view of the Branchial arches of Heptanchus. (From Gegenbaur).

1. basi-hyal.	7. pharyngo-branchial, common
2. cerato-hyal.	to the sixth and seventh
3. second hypo-branchial.	arches.
4. first cerato-branchial.	8. basibranchial of second arch.
5. first epi-branchial.	9. basibranchial, common to the
6. first pharyngo-branchial.	sixth and seventh arches.

3. The branchial arches, generally five in number, all of which except the last bear branchiostegal rays. In the Notidanidae the number of branchial arches is increased beyond the normal series, thus in Hexanchus there are six, and in Heptanchus seven. There are six also in Chlamydoselache and Protopterus.

4. The so-called external branchial arches which are cartilaginous rods attached to all the visceral arches. They are especially large in Cestracion.

The skull in Holocephali is entirely cartilaginous. The palato-pterygo-quadrate bar is fixed to the cranium, and to it the mandible articulates. There is a well-marked joint between the skull and the spinal column.

In living Cartilaginous Ganoids the primitive cartilaginous cranium is very massive, and is greatly prolonged anteriorly, while posteriorly it merges into the spinal column. Although it is mainly cartilaginous a number of ossifications take place in the skull, and membrane bones are now found definitely developed, especially in connection with the roof of the cranium. In Acipenser (fig. 18) the ossifications in the cartilage include the pro-otic, which is pierced by the foramen for the fifth nerve, the alisphenoid, orbitosphenoid, ectethmoid, palatine, pterygoid, meso-pterygoid, hyomandibular (fig. 18, 11), cerato-hyal, all the cerato-branchials, and the first two epi-branchials. Most of these structures are, however, partly cartilaginous, though they include an ossified area. The membrane bones too of Acipenser are very well developed, they include a bone occupying the position of the supra-occipital, and form a complete dorsal cephalic shield. Resting on the ventral surface are a vomer and a very large parasphenoid (fig. 18, 3). There is a bony operculum attached to the hyomandibular, and membrane bones representing respectively the maxilla and dentary are attached to the jaws. The suspensorium is most markedly hyostylic. The palato-pterygo-quadrate bar has a very curious shape and is quite separate from the cranium. It is connected to the hyomandibular by a thick symplectic ligament containing a small bone homologous with the symplectic of Teleosteans.

Polyodon differs much from Acipenser, the membrane bones not being so well developed though they cover the great cartilaginous snout.

Fig. 18. Lateral view of the skull of a Sturgeon (Acipenser sturio). Nearly all the membrane bones have been removed (Brit. Mus.).

1. nasal cavity.	10. palatine.
2. orbit.	11. hyomandibular.
3. parasphenoid.	12. pharyngo-branchial.
4. vomer.	13. epi-branchial.
5. pterygoid.	14. cerato-branchial.
6. maxilla. (The dotted line	15. hypo-branchial.
running from 6 passes	16. coalesced anterior vertebrae.
into the mouth cavity.)	17. inter-hyal.
7. dentary.	18. cerato-hyal.
8. symplectic.	19. rib.

The skull in Polypterus (Crossopterygii) shows a great advance towards the condition met with in Teleostei. The cranium remains to a great extent unossified, and large dorsal and ventral fontanelles pierce its walls. It is covered by a great development of membrane bones, paired nasals, frontals, parietals, supra- and post-temporals, and dermo-supra-occipitals among others being present. The palato-pterygo-quadrate bar is fused to the cranium, and in connection with it the following paired membrane bones appear, palatine, ecto-, meso- and meta-pterygoid, and further forwards jugal, vomer, maxilla and premaxillae. The membrane bones developed in connection with each ramus of the mandible are the dentary, angular, and splenial, in addition to the cartilage bone the articular. Several large opercular bones occur. There are also a pair of large jugular or gular plates, and several large opercular bones.

In Bony Ganoids both cartilage bone and membrane bone is well developed. The pro-otics and exoccipitals are well ossified, but the supra-occipital and pterotics are not. Lateral ethmoids are developed, and there are ossifications in the sphenoidal region which vary in different forms. The place of the cartilaginous palato-pterygo-quadrate is taken by a series of bones, the quadrate behind and the palatine, ecto-, meso- and meta-pterygoids in front. In Lepidosteus, however, the palatine and pterygoid are membrane bones, as they are in Polypterus and the Frog. Paired maxillae, premaxillae, vomers and a parasphenoid occur forming the upper jaw and roof of the mouth, and a series of membrane bones are found investing the mandible and forming the operculum.

In Amia[45] membrane bones are as freely developed as they are in Teleosteans; they include on each side a squamosal, four opercular bones, a lachrymal, a pre-orbital, one or two suborbitals, two large postorbitals and a supratemporal; while investing the mandible, besides the dentary, splenial, angular, and supra-angular, there is an unpaired jugular. The articular too is double and a mento-meckelian occurs. In Amia teeth are borne on the premaxillae, maxillae, vomers, palatines and pterygoids.

Bony Ganoids are the lowest animals in which squamosal bones are found, and they do not occur in Teleosteans.

The suspensorium in bony Ganoids, as in the Chondrostei, is hyostylic, and there are two ossifications in the hyomandibular cartilage, viz. the hyomandibular, and the symplectic.

The skull of Teleostei is very similar to those of Lepidosteus and Amia. Although the bony skull is greatly developed and very complicated, much of the original cartilaginous cranium often persists. Membrane bones are specially developed on the roof of the skull where they include the parietal, frontal, and nasal bones. The same bones are developed in connection with the upper jaw and roof of the mouth as in bony Ganoids, but only two membrane bones occur in the lower jaw, viz. the angular and dentary. A number of large ossifications take place in the cartilage of the auditory capsules. In some forms parts of the last pair of branchial arches are broadened out and form the pharyngeal bones which bear teeth. The opercular bones and those of the upper and lower jaws are quite comparable to those of bony Ganoids.

A full account of the Teleostean skull has been given in the case of the Salmon (pp. 87-96) and the Cod (pp. 96-101).

In Dipnoi the skull is mainly cartilaginous, but both cartilage- and membrane-bone occur also. Cartilage-bone is found in the ossified exoccipitals, while of membrane-bones Protopterus has among unpaired bones a fronto-parietal, a median ethmoid, and a parasphenoid, and among paired bones nasals and large supra-orbitals. The skull of Ceratodus (fig. 19) has an almost complete roof of membrane bones, including some whose homology is doubtful. The ethmo-vomerine region is always cartilaginous, but bears small teeth. The palato-pterygo-quadrate bar is ossified and firmly united to the cranium, and the mandible articulates directly with it (autostylic). Membrane bones are freely developed in connection with the mandible, dentary, splenial, and angular bones being all present. There are two opercular bones.

In the extinct Dipteridae the cranium is very completely covered with plates of dermal bone, and the skeleton in general is more ossified than is the case in recent Dipnoi.

Six pairs of branchial arches occur in Protopterus; Ceratodus and Lepidosiren have five, like most other fish. The branchial arches bear gill rakers.

Fig. 19. Dorsal (to the left) and ventral (to the right) views of the cranium of Ceratodus miolepis (after Günther).

1. cartilaginous part of the	7. pre-opercular (squamosal).
quadrate with which the	8. second rib.
mandible articulates.	9. first rib.
2. scleroparietal.	10. vomerine tooth.
3. frontal.	11. palato-pterygoid tooth.
4. ethmoid.	12. palato-pterygoid.
5. nares.	13. parasphenoid.
6. orbit.	14. interopercular.

Ribs.

As has been already mentioned (p. 24), although ribs commonly appear to be the cut-off ends of the transverse processes, they are really elements derived from the ventral or haemal arch.

In Elasmobranchii and other cartilaginous fish they have the form of small cartilaginous structures imperfectly separated from the diverging halves of the ventral arch, and are often absent.

In Teleostei and bony Ganoids they often have different attachments in different parts of the body. In the tail region they are not differentiated from the two halves of the ventral arch, which meet in the middle line, and are prolonged into a haemal spine. In the posterior trunk region they sometimes form distinct processes diverging from the two halves of the ventral arch; while further forward they may shift their attachment so as to arise from the dorsal side of the two halves of the ventral arch and at some distance from their ends, which now diverge as ventri-lateral processes.

Appendicular Skeleton.

Pectoral girdle.

The simplest type of pectoral girdle is found in Elasmobranchs. It is entirely cartilaginous and consists of a curved ventrally-placed rod, ending dorsally in two horn-like scapular processes which are sometimes attached to the cranium or vertebral column. In Rays the shoulder girdle is very large, and has a distinct suprascapular portion forming a broad plate attached to the neural spines of the vertebrae. There is often a cup-like glenoid cavity for the articulation of the limb; this cavity is specially large in Rays and is much pierced by holes. In Dipnoi the cartilaginous girdle still occurs, but on it there is a deposit of membrane bone forming the clavicle, infraclavicle, and supra-clavicle. These bones, which with the exception of the clavicle, are unknown in higher vertebrates, are better developed in Ganoids, and best of all in Teleosteans. They are connected by the supratemporal with the epi-otic and opisthotic regions of the cranium. Owing to this development of dermal bone, the original cartilaginous arch becomes much reduced, but ossifications representing the scapula and coracoid occur in bony Ganoids and Teleosteans.

Pelvic girdle.

In Elasmobranchs the pelvic girdle consists of a short ventral rod of cartilage representing the ischium and pubis, which does not send up dorsal iliac processes. In Chimaera the pelvic girdle has a flattened pointed iliac portion, and ventrally an unpaired movable cartilaginous plate which bears hooks and is supposed to be copulatory in function. Claspers of the usual type are present as well. The Dipnoi have a primitive kind of pelvis in the form of a cartilaginous plate lying in the mid-ventral line and drawn out into three horns anteriorly. In Ganoids the pelvis has almost entirely disappeared, though small cartilaginous vestiges of it remain in Polypterus. In Teleosteans even these vestiges are gone, and in these fish and Ganoids the place of the pelvis is taken by the enlarged basi-pterygia (meta-pterygia) of the fins.

Paired fins[46].

As regards the origin of the limbs or paired fins of fishes there are two principal views. One view, that of Gegenbaur, considers that limbs and their girdles are derived from visceral arches which have migrated backwards. The other view, which probably now has the greater number of supporters, considers that the paired fins of fishes are of essentially the same nature as the median fins.

According to Gegenbaur's view[47] the archipterygium of Ceratodus (fig. 20) represents the lowest type of fin; it consists of a central cartilaginous axis bearing a large number of radiale. The dorsal or pre-axial radiale are more numerous than the ventral or postaxial, and at the margin of the fin[48] the cartilaginous endoskeletal radiale are replaced by horny exoskeletal fin-rays.

Fig. 20. Lateral view of the skeleton of Ceratodus miolepis (after Günther).

1. ethmoid.	9. pectoral girdle.
2. scleroparietal.	10. proximal cartilage of pectoral fin.
3. frontal.	11. pectoral fin.
4. cartilaginous posterior part of cranium.	12. pelvic girdle.
5. pre-opercular (squamosal).	13. pelvic fin.
6. opercular.	14. spinal column.
7. suborbital.	15. caudal fin (diphycercal).
8. orbit.

It is impossible here to give a full discussion of the rival views, but some of the points which support Gegenbaur's view may be mentioned. The fact that migration of visceral arches has to be assumed is no difficulty, as it is obvious that migration in the opposite direction has taken place in many Teleosteans such as the Cod, whose pelvic fins are attached to the throat in front of the pectorals. If migration did take place, the pelvic fins being older than the pectoral should be the more modified, and this is the case. Again, if the pectoral girdle is a modified branchial arch, it must at some period have carried a gill, and in Protopterus it does bear a vestigial gill.

According to the view more prevalent at the present time, the paired fins have been derived from two continuous folds of skin and their skeletal supports running forward from the anal region along the sides of the body, their character being similar to the fold that gave rise to the median fins. In support of this view it may be argued that the paired and unpaired fins are often identical in structure, and that some Elasmobranch embryos do show a ridge running between the pectoral and pelvic fins. Then from this continuous fold two pairs of smaller folds may have been specialised off, and in each a number of cartilaginous radiale may have been developed. The fin of Cladoselache from the Carboniferous of Ohio apparently illustrates this condition. It consists of certain basal pieces which do not project beyond the body wall and bear a number of unsegmented cartilaginous radiale, which show crowding together and are sometimes bifurcated distally; they extend throughout the whole fin from the body wall to the margin. From this fin the archipterygium might be easily derived by the enlargement of one of the middle radiale and the segmentation and partial fusion of them all.

Whether the archipterygium be a primitive or secondary type of fin, when it is once reached it is easy to derive all the other types from it. The fins of the other living Dipnoi,—Protopterus and Lepidosiren—are simply archipterygia from which the radiale have almost or completely disappeared, leaving only the segmented axes. Archipterygia too are found in the pectoral fins of the Ichthyotomi, but the postaxial radiale are much reduced.

The ichthyopterygium, or type of fin, characteristic of many modern Elasmobranchs such as Scyllium, may have been derived from the archipterygium by the gradual reduction of the rays on the postaxial side of the axis and their condensation on the pre-axial side. The Ichthyotomi such as Xenacanthus show one stage in the reduction of the postaxial rays, and a further stage is seen in the Notidanidae and some other sharks like Scymnus and Acanthias, in which a few postaxial rays still remain. The condensation of the pre-axial rays when further continued leads to one of the rays getting an attachment to the girdle. Thus the fin comes to articulate with the girdle by two basalia or basal pieces; a third attachment is formed in the same way and the three basalia are called respectively pro-, meso-, and meta-pterygia. By some authors the meta-pterygium and by others the meso-pterygium is regarded as homologous with the axis of the archipterygium.

The pectoral fins of Elasmobranchs vary very much in their mode of attachment. In some of the sharks, including the Notidanidae and Scyllium, all three basalia articulate with the pectoral girdle, while in others such as Cestracion the meta-pterygium is excluded. In Rays the propterygium and the meta-pterygium are long and narrow and diverge much from one another; other basalia work their way in between the meso-pterygium and meta-pterygium, and come to articulate with the pectoral girdle. Sometimes they fuse and form a second meso-pterygium. The radiale are greatly elongated and are segmented.

In Chimaera all three basalia are present, but the meso-pterygium is shifted and does not articulate with the pectoral girdle[49].

In Acipenser and Polyodon the pectoral fin is built on the same type as in Elasmobranchs, but becomes modified from the fact that the propterygium is replaced by dermal bone which forms a large marginal ray. Extra meso-pterygia are formed in the same way as in Rays.

In Polypterus the pro-and meta-pterygia have ossified while the meso-pterygium remains chiefly cartilaginous; the fin-rays are also chiefly ossified.

In Amia, Lepidosteus, and certain Teleosteans like Salmo, not only the propterygium but the meso-pterygium is almost suppressed by the marginal ray.

In the great majority of Teleosteans a still further stage is reached, the endoskeletal elements, the basalia and radiale are almost entirely suppressed and the fin comes to consist entirely of ossified fin-rays of dermal origin.

In some Teleosteans—Exocaetus, a herring, and Dactylopterus, a gurnard—the pectoral fins are so enormously developed that by means of them the fish is able to fly through the air for considerable distances. The skeleton of these great fins is almost entirely composed of dermal bone.

Pelvic fin.

The pelvic fin is almost always further removed from the archipterygial condition, and is in general more modified than is the pectoral. Thus in the Ichthyotomi, while the pectoral fins are archipterygia similar to those of Ceratodus, the pelvic fins consist of an axis bearing rays on the postaxial side only, and prolonged distally into a clasper. In Dipnoi however the pelvic fins are very similar to the pectoral. In Elasmobranchs the meso-pterygium is missing, the propterygium is small or absent, and the fin is mainly composed of the meta-pterygium (generally called basi-pterygium) and its radiale. The males in Elasmobranchii and Holocephali have the distal end of the meta-pterygium prolonged into a clasper.

In Ganoids and in Teleosteans the loss of the pelvic girdle causes the pelvic fin to be still further removed from the primitive state. There is always a large basi-pterygium which lies imbedded in the muscles and meets its fellow at its proximal end. In Cartilaginous Ganoids it has a secondary segmentation. Its relation to its fellow is subject to much variation in Teleosteans, sometimes as in the Perch the two are in contact throughout, sometimes as in the Salmon they meet distally as well as proximally, but are elsewhere separated by a space, sometimes as in the Pike and Bony Ganoids they diverge widely. The radiale are articulated to the basi-pterygium. In Cartilaginous Ganoids and Polypterus they are well developed, in other Ganoids and in Teleosteans they are in the main replaced by dermal fin-rays.

In some Teleosteans such as the Cod the pelvic fins have migrated from their usual position and come to be attached to the throat in front of the pectoral fins. Fish with this arrangement are grouped together as jugulares.

Amphibia differ markedly from Pisces in the fact that in the more abundant and familiar forms the skin is naked, and that when the integument is prolonged into median fins they are devoid of fin-rays. The notochord may persist, but bony vertebral centra are always developed. These are sometimes biconcave, sometimes procoelous, sometimes opisthocoelous. There is only one sacral vertebra, except in rare cases. The cartilaginous cranium persists to a considerable extent but is more or less replaced by cartilage bone, and overlain by membrane bone. The basi-occipital is not completely ossified, and the skull articulates with the vertebral column by means of two occipital condyles formed by the exoccipitals.

There is a large parasphenoid, but there are no ossifications in the basisphenoidal, presphenoidal, and alisphenoidal regions. In most cases the epi-otics and opisthotics are ossified continuously with the exoccipitals.

The palato-pterygo-quadrate bar is firmly united to the cranium, so the skull is autostylic. The palatines and pterygoids are membrane bones. Teeth are nearly always borne on the vomers and commonly on the maxillae and premaxillae. There are no sternal ribs, and the sternum is very intimately related to the pectoral girdle. There are no obturator foramina. The limbs are as in the higher vertebrata, divisible into upper arm, fore-arm, and manus (wrist and hand), and into thigh, shin, and pes (ankle and foot) respectively. The posterior limb is, as a rule, pentedactylate, but in nearly every case the pollex is vestigial or absent.

Order 1. Urodela[51].

The Urodela are elongated animals with a naked skin, a persistent tail, and generally four short limbs.

The vertebral centra are opisthocoelous or biconcave, and there are numerous precaudal vertebrae. Portions of the notochord commonly persist in the intervertebral spaces. In the skull there is no sphenethmoid forming a ring encircling the anterior end of the brain, its place being in many cases partly taken by a pair of orbitosphenoids. There is no quadratojugal, and the quadrate is more or less ossified. The mandible has a distinct splenial, and the articular is ossified.

There is no definite tympanic cavity. The hyoid apparatus is throughout life connected to the quadrate by ligament, and a large basilingual plate does not occur. The ribs are short structures with bifurcated proximal ends. In the pelvis the pubis remains cartilaginous, and there is a bifid cartilaginous epipubis. The bones of the fore-arm and shin remain distinct, and the manus never has more than four digits.

Suborder (1). Ichthyoidea.

The vertebrae are amphicoelous, but the notochord remains but little constricted throughout the whole length of the vertebral column. Three or four branchial arches nearly always persist in the adult. The cartilages of the carpus and tarsus remain unossified.

The Ichthyoidea may be subdivided again into two groups:—

A. Perennibranchiata, whose chief distinguishing skeletal characters are that the skull is elongated, the premaxillae are not ankylosed, the maxillae are vestigial or absent; there are sometimes no nasals, and the palatines bear teeth;

e.g. Siren, Proteus, Menobranchus.

B. Derotremata, whose chief distinguishing skeletal characters are that there are large maxillae and nasals; teeth are borne by both maxillae and premaxillae; there are no palatines; and both pectoral and pelvic limbs are always present;

e.g. Amphiuma, Megalobatrachus, Cryptobranchus.

Suborder (2). Salamandrina.

The vertebrae are opisthocoelous. The skull is broad, and teeth are borne by both premaxillae and dentaries. Nasal bones are present. The remains of only two branchial arches are found in the adult. The carpus and tarsus are more or less ossified.

This suborder includes the Newts (Molge), Salamanders (Salamandra), and Amblystoma.

Order 2. Labyrinthodontia[52].

These are extinct Amphibia with a greatly developed dermal exoskeleton, which is generally limited to the ventral surface. The body and tail are long and in some cases limbs are absent. The teeth are pointed and often have the dentine remarkably folded. The vertebrae are amphicoelous, and are generally well ossified. The skull is very solid, and has a greatly-developed secondary roof which hides the true cranium and is very little broken up by fossae. Paired dermal supra-occipitals are found, and there is an interparietal foramen. The epi-otics and opisthotics form a pair of bones distinct from the exoccipitals. Four simple limbs of moderate length are generally present, and in some cases all four limbs are pentedactylate. Among the better known genera of Labyrinthodonts are Mastodonsaurus, Nyrania, and Archegosaurus.

Order 3. Gymnophiona[53].

These animals form a group of abnormal worm-like Amphibia having an exoskeleton in the form of subcutaneous scales arranged in rings. The vertebrae are biconcave and are very numerous; very few however belong to the tail. The skull has a complete secondary bony roof, the mandible bears teeth and has an enormous backward projection of the angular. The hyoid arch has very slender cornua and no distinct body, it is attached neither to the cranium nor to the suspensorium. The ribs are very long and there are no limbs or limb girdles.

Order 4. Anura.

These are tailless Amphibia, which except in a few instances, are devoid of an exoskeleton. The vertebrae are as a rule procoelous, and are very few in number. The post-sacral part of the spinal column ossifies continuously, forming an unsegmented cylindrical rod, the urostyle. Remains of the notochord persist, lying vertebrally, i.e. enclosed within the centra of the several vertebrae, and not as in Urodela lying between one vertebra and the next. The skull is very short and wide. The mandible is almost always, if not invariably, toothless.

The frontals and parietals on each side are united so as to form a pair of fronto-parietals, and a girdle-like sphenethmoid is present.

The quadrate is not generally ossified. A predentary or mento-meckelian bone is commonly present in the mandible, and a single bone represents the angular and splenial. The branchial arches are much reduced in the adult, and the distal ends of the cornua unite to form a flat basilingual plate of a comparatively large size.

Ribs are very little developed. Clavicles are present. The ilia are very greatly elongated. The anterior limb has four well-developed digits and a vestigial pollex, and is of moderate length; the radius and ulna have fused. The posterior limb is greatly elongated and is pentedactylate; the tibia and fibula are fused, while the calcaneum and astragalus are greatly elongated, and it is largely owing to them that the length of the limb is so great. The group includes the Frogs and Toads, the predominant Amphibia of the present time.

I. EXOSKELETON.

The skin of the Newt is quite devoid of any exoskeletal structures. The only exoskeletal structures that the animal possesses are the teeth, and these are most conveniently described with the endoskeleton.

II. ENDOSKELETON.

The endoskeleton of the Newt, though ossified to a considerable extent, is more cartilaginous than is that of the frog. It is divisible into an axial portion including the vertebral column, skull, ribs, and sternum, and an appendicular portion including the skeleton of the limbs and their girdles.

1. The Axial Skeleton.

A. The Vertebral column.

This consists of about fifty vertebrae arranged in a regular continuous series. The first vertebra differs a good deal from any of the others; the seventeenth or sacral vertebra and the eighteenth or first caudal also present peculiarities of their own. The remaining vertebrae are divided by the sacrum into an anterior series of trunk vertebrae which bear fairly large ribs, and a posterior series of caudal vertebrae, all of which except the first few are ribless.

The trunk vertebrae.

Any vertebra from the second to the sixteenth may be taken as a type of the trunk vertebrae.

The general form is elongated and somewhat hour-glass shaped, and the centra are convex in front and concave behind; an opisthocoelous condition such as this is quite exceptional in Anura. The notochord may persist intervertebrally[54], but in the centre of each vertebra it becomes greatly constricted or altogether obliterated, and replaced by marrow. The superficial portion of the centrum is ossified, while the articular surfaces are cartilaginous. The neural arches are low and articulate together by means of zygapophyses borne on short diverging processes. The anterior zygapophyses look upwards, the posterior downwards. Each neural arch is drawn out dorsally into a very slight cartilaginous neural spine.

On each centrum, at a little behind the middle line, there arise a pair of short backwardly-directed transverse processes; each of which becomes divided into two slightly divergent portions, a dorsal portion which meets the tubercular process of the rib and is derived from the neural arch, and a ventral portion which meets the capitular process of the rib and is derived from the ventral or haemal arch. The division between these two parts of the transverse processes can be traced back as far as the sacrum.

The first vertebra as already mentioned differs much from all the others. It has no ribs, and presents anteriorly two slightly divergent concave surfaces which articulate with the occipital condyles of the skull. Between these surfaces the dorsal portion of the anterior face of the centrum is drawn out into a prominent odontoid process, the occurrence of which renders it probable that the first vertebra of the newt is really the axis, and that the atlas with the exception of the odontoid process has become fused with the skull. The sacral vertebra or sacrum differs from the vertebrae immediately in front of it only in the fact that its transverse processes are stouter and more obviously divided into dorsal and ventral portions.

The caudal vertebrae.

The caudal vertebrae are about twenty-four in number. The anterior ones have hour-glass shaped centra, and short backwardly-directed transverse processes. The middle and posterior ones have rather shorter centra, and are without transverse processes. The neural arches resemble those of the trunk vertebrae, but each is drawn out into a rather high cartilaginous neural spine abruptly truncated anteriorly. All the caudal vertebrae except the first have also a haemal arch, which is very similar to the neural arch, and is drawn out into a haemal spine quite similar to the neural spine. Both neural and haemal arches are ossified continuously with the centra.

B. The Skull.

The skull of the newt is divisible into three principal parts:—

(1) an axial part, the cranium proper, which encloses the brain and to which

(2) the capsules of the auditory and olfactory sense organs are fused;

(3) the skeleton of the jaws and hyoid apparatus. The skull is much flattened and expanded, though not so much as in the frog.

(1) The cranium proper.

The cranium proper or brain case is an unsegmented tube which remains partly cartilaginous, and is partly converted into cartilage bone, partly sheathed by membrane bone. The roof and floor of the cartilaginous cranium are, as is the case also in the frog, pierced by holes or fontanelles, and these are so large that the main part of the roof and floor comes to be formed by membrane bone.

Two pairs of large ossifications take place in the cranial walls. Of these the more posterior on each side represents the exoccipital and all three periotic bones. It bears a small convex patch of cartilage for articulation with the atlas, and with its fellow forms the boundary of the foramen magnum.

Two foramina pierce the exoccipital just in front of the occipital condyle and transmit respectively the glossopharyngeal and pneumogastric (fig. 21, X) nerves. Lying laterally to these nerve openings is seen a patch of cartilage, the stapes, which is homologous with the stapes or proximal element of the columellar chain in the frog. Further forward in front of the stapes is the small opening for the exit of the facial nerve, and seen in a lateral view close to the orbitosphenoid, that for the trigeminal (fig. 21, C, 5).

In front of these large bones the lateral parts of the cranial walls remain cartilaginous for a short distance, and then there follow two elongated bones, the orbitosphenoids (fig. 21, B and C, 11), pierced by the foramina for the exit of the optic nerves. These bones partly correspond to the sphenethmoid of the frog.

The membrane bones connected with the cranium are the parietals, frontals and prefronto-lachrymals on the dorsal surface, and the parasphenoid on the ventral surface.

The parietals (fig. 21, A and C, 6) roof over the posterior part of the great dorsal fontanelle and overlap the exoccipito-periotics. They meet one another along a sinuous suture in the middle line, as do also the frontals which overlap them in front. The frontals and parietals both extend for a short distance down the sides of the cranium and meet the orbitosphenoids. The prefronto-lachrymals (fig. 21, A and C, 7) connect the frontals with the maxillae.

Fig. 21. A dorsal, B ventral, and C lateral views of the skull
of a Newt (Molge cristata) × 2½ (after Parker).
The cartilage is dotted, the cartilage bones are marked with dots and
dashes, the membrane bones are left white.

1. premaxillae.	14. pro-otic region of
2. anterior nares.	exoccipito-periotic.
3. posterior nares.	15. quadrate.
4. nasal.	16. quadrate cartilage.
5. frontal.	17. exoccipital region of
6. parietal.	exoccipito-periotic.
7. prefronto-lachrymal.	18. articular.
8. maxillae.	19. articular cartilage.
9. vomero-palatine.	20. dentary.
10. parasphenoid.	21. splenial.
11. orbitosphenoid.	22. middle narial passage.
12. pterygoid.	II. V. VII. IX. X. foramina for
13. squamosal.	the exit of cranial nerves.

On the ventral surface is the large parasphenoid (fig. 21, B, 10), which is widest behind and overlapped anteriorly by the vomero-palatines.

(2) The sense capsules.

The auditory capsules become almost completely ossified continuously with the exoccipitals; they have been already described.

The nasal capsules are large and quite unossified though they are overlain by membrane bone. They appear on the dorsal surface between the anterior nares and the nasal process of the premaxillae. They enclose the nasal organs, bound the inner side of the anterior narial opening, and are connected with one another posteriorly by a cartilaginous area.

Developed in connection with the nasal capsules are a pair of rather large nasals (fig. 21, A and C, 4), which lie on the dorsal surface immediately in front of the frontals. Each forms part of the posterior boundary of one of the anterior nares, and the two are separated from one another in the middle line by the nasal process of the premaxillae (fig. 21, A, 1), and the opening of the middle narial passage (fig. 21, A and B, 22), which passes right through the skull.

On the ventral surface of the skull and forming the greater part of the boundary of the posterior nares are two large bones, the vomero-palatines (fig. 21, B and C, 9). Each consists of a wide anterior portion, partly separated from its fellow in the middle line by the ventral opening of the middle narial passage, and of a long pointed posterior portion which is separated from its fellow by the parasphenoid, and bears a row of small pointed teeth formed of dentine capped with enamel.

(3) The jaws.

The upper jaw of the newt is a discontinuous structure divided into two parts, an anterior part which consists of membrane bones, the maxillae and premaxillae, and a posterior part which remains mainly cartilaginous.

The premaxillae are united, forming a single bone, which in a ventral view is seen to meet the maxillae and vomero-palatines, and in a dorsal view to send back a nasal process (fig. 21, A, 1) between the nasals.

The maxillae are large bones, each terminating in a point posteriorly. A single row of teeth similar to those on the vomero-palatines runs along the outer margin of the maxillae and premaxillae.

The posterior part of the upper jaw forms a mass of cartilage which extends forwards towards the maxillae as a long pointed process whose ventral surface and sides are overlapped by a membrane bone, the pterygoid (fig. 21, 12).

The suspensorial bones include the quadrate and squamosal. The quadrate (fig. 21, 15) which forms the true suspensorium is directed forwards and outwards, and is terminated by a patch of cartilage with which the mandible articulates.

The lower jaw or mandible remains partly cartilaginous, while its ossifications include two membrane bones and one cartilage bone. The cartilage bone is the articular (fig. 21, C, 18), it forms the posterior part of the ramus, extends forwards for some distance along its inner side, and is terminated posteriorly by a patch of cartilage which articulates with the quadrate. The dentary (fig. 21, C, 20) is a large bone which forms the anterior part and nearly all the outer half of each ramus, and bears teeth similar to those of the upper jaw. Attached to its inner face is a long slender splenial (fig. 21, C, 21).

The Hyoid apparatus.

This consists of the hyoid arch and part of the first two branchial arches.

The hyoid arch (fig. 29, A, 2) consists of a pair of cornua, each of which is divided into two halves. The dorsal half forming the cerato-hyal is mainly ossified though tipped with cartilage, and is connected by ligament with the suspensorium. The ventral half (hypo-hyal) is cartilaginous, and is connected with the basibranchial.

The branchial arches consist of a median piece, the basibranchial, which is ossified in the centre and cartilaginous at either end, and of two pairs of cerato-branchials which are attached to the cartilaginous part (fig. 29, A, 8) of the basibranchial. The first cerato-branchial is chiefly ossified, the second (fig. 29, A, 4) is a good deal smaller and is cartilaginous. Both are united dorsally to a single epi-branchial, which is terminated by a small cartilaginous area at the free end but is elsewhere well ossified.

C. The Ribs.

The ribs are short imperfectly ossified structures, bifid at their proximal end where they articulate with the transverse processes, and tipped both proximally and distally with cartilage. The dorsal portion of the proximal end corresponds to the tuberculum of the ribs of higher animals, and the ventral portion to the capitulum. Some of the anterior ribs have a step-like notch on their dorsal surfaces.

The second to twelfth ribs are fairly equal in size, but further back they decrease slightly. The ribs which connect the sacral vertebrae with the ilia are however large. The short ribs borne on the anterior caudal vertebrae are cartilaginous.

D. The Sternum.

The sternum (fig. 22, A, 6) is a rather broad plate of cartilage, drawn out posteriorly into a median process marked by a prominent ridge. On its antero-lateral margins it bears surfaces for articulation with the pectoral girdle.

2. The Appendicular Skeleton.

A. The Pectoral girdle.

This is of a very simple character, and remains throughout life in an imperfectly ossified condition. It consists of a dorsal scapular portion, and a ventral coracoid portion partially divided into an anterior part, the precoracoid, and a posterior part, the coracoid.

Fig. 22. A ventral, and B lateral view of the shoulder girdle and sternum of an old male Crested Newt (Molge cristata) × 3 (after Parker).

1. scapula.	4. glenoid cavity.
2. suprascapula.	5. precoracoid.
3. coracoid.	6. sternum.

The scapular portion is a slightly curved oblong plate; its proximal third the scapula (fig. 22, 1) is ossified and bounds part of the well-marked glenoid cavity (fig. 22, 4); its distal portion forms a large oblong cartilaginous plate, the suprascapula (fig. 22, 2).

The precoracoid (fig. 22, 5) forms a small forwardly-directed cartilaginous plate. The coracoid (fig. 22, 3) forms a much larger plate, the greater part of which is unossified and overlaps its fellow in the middle line, the two being overlapped by the sternum. Around the glenoid cavity is an area which is mainly ossified and is continuous with the scapula.

B. The Anterior limb.

This is divisible into three parts, the upper arm or brachium, the fore-arm or antibrachium, and the manus.

The upper arm includes a single bone, the humerus.

The humerus is a slender bone cylindrical in the middle and expanded at either end, the proximal part forms a rounded head which articulates with the glenoid cavity. Along the proximal part of the anterior or pre-axial surface runs a strong deltoid ridge. The proximal part of the postaxial surface also bears a small outgrowth.

The fore-arm contains two bones, the radius and ulna, both of which are small and imperfectly ossified at their terminations.

The radius (fig. 23, B, 11) or pre-axial bone is rather the larger of the two, and is considerably expanded at its proximal end. The ulna or postaxial bone is somewhat expanded distally, but is not drawn out proximally into an olecranon process.

The manus consists of two parts, a group of small bones forming the carpus or wrist, and the hand.

The carpus is in a very simple unmodified condition as compared with that of the Frog. It consists of a proximal row of two bones and a distal row of four, with one, the centrale, interposed between. All these bones are small and polygonal and are imbedded in a plate of cartilage.

The bones of the proximal row are a smaller pre-axial bone, the radiale (fig. 23, B, 13), and a larger postaxial bone, which represents the fused ulnare and intermedium of the very simple carpus described on pp. 26 and 27.

The four bones of the distal row are respectively carpalia 2, 3, 4 and 5.

The hand consists of four digits, that corresponding to the thumb of the human hand, judging from the analogy of the frog probably being the one that is absent.

Fig. 23. A right posterior, and B right anterior limb of a Newt × 1½ (Molge cristata).

1. femur.	10. humerus.
2. tibia.	11. radius.
3. fibula.	12. ulna.
4. tibiale.	13. radiale.
5. intermedium.	14. intermedium and ulnare fused.
6. fibulare.	15. centrale of carpus, the pointing
7. centrale of tarsus.	line passes across carpale 2.
8. tarsale 1.	16. carpale 3.
9. tarsalia 4 and 5 fused.	17. carpale 5.
I. II. III. IV. V. digits.

Each digit consists of a somewhat elongated metacarpal and of two or three phalanges. The metacarpals are contracted in the middle and expanded at either end. They are connected with the carpus by cartilage, and the articulations between the several phalanges, and between the metacarpals and phalanges are also cartilaginous. The second, third, and fifth digits have two phalanges apiece, the fourth, which is the longest, has three. The second metacarpal in the specimen examined and figured articulates partly with carpale 2, partly with carpale 3.

C. The Pelvic girdle.

The pelvic girdle of the Newt is in a much less modified condition than is that of the Frog (see p. 165). It consists of a dorsal element, the ilium, a posterior ventral element, the ischium, and an anterior ventral element, the pubis, to which is attached an epipubis.

The ilium is a somewhat cylindrical bone which at its ventral end meets the ischium, and forms part of the acetabulum. It is then directed upwards and slightly backwards, and is attached to the ribs of the sacral vertebra.

The ischia are a pair of somewhat square bones which meet one another in the middle line; they form part of the acetabulum, and are united to the ilia above.

In front of the ischia is a narrow cartilaginous area which represents the pubes. Projecting forwards from it is a bifid cartilaginous epipubis.

D. The Posterior limb.

This is divisible into a proximal portion, the thigh, a middle portion, the crus or shin, and a distal portion, the pes.

The thigh consists of a single bone, the femur (fig. 23, A, 1), which has a thin shaft and expanded ends. The anterior part of the pre-axial border and posterior part of the postaxial border bear slight outgrowths.

The crus or shin includes two short bones, the tibia and fibula, which are nearly equal in length. The pre-axial bone or tibia is a straight bone thickest at its proximal end, the postaxial bone or fibula (fig. 23, A, 3) is a rather stouter curved bone of nearly equal diameter throughout.

The pes includes the tarsus or ankle, and the foot.

The tarsus consists of eight small bones arranged in a proximal row of three, the tibiale, intermedium and fibulare, and a distal row of four tarsalia, with one bone, the centrale (fig. 23, A, 7), interposed between the two rows. In the specimen examined, the tibiale, is a small bone articulating with the tibia, the intermedium (fig. 23, A, 5) is larger and articulates with both tibia and fibula, the fibulare is the largest of the three and articulates with the fibula.

The bones of the distal row are tarsalia 1, 2, 3, and a bone representing 4 and 5 fused. In the specimen examined tarsale 1 is pushed away dorsally (fig. 23, A, 8), so as to lie between the tibiale and tarsale 2. All the tarsal bones are small and somewhat polygonal, and are connected with one another, and with the tibia and fibula on the one hand, and with the metatarsals on the other by a thin layer of cartilage.

The five digits of the foot each consist of a metatarsal and of a certain number of phalanges. In the specimen examined, owing to the shifting of tarsale 1, the first metatarsal as well as the second articulates with tarsale 2, while the fifth metatarsal articulates partially with the bone representing the fused tarsalia 4 and 5, partially with the fibulare. All the bones of the digits except the distal phalanges are terminated at each end by cartilaginous epiphyses, the distal phalanx of each digit has a cartilaginous epiphysis only on its proximal end.

The first, second, and fifth digits have two phalanges apiece, the third and fourth have three.

Figure 31 B, showing a Newt's tarsus copied from Gegenbaur, has precisely the arrangement generally regarded as primitive for the higher vertebrates, except that tarsalia 4 and 5 are fused.

I. EXOSKELETON.

The skin of the frog is smooth and quite devoid of scales or other exoskeletal structures. The only exoskeletal structures met with in the frog are:—

1. The teeth, which are most conveniently described with the endoskeleton.

2. The horny covering of the calcar or prehallux (see p. 167).

II. ENDOSKELETON.

The endoskeleton of the adult frog consists partly of cartilage, partly of bone and each of these types of tissue occurs in two forms. The cartilage may be hyaline, as in the omosternum and xiphisternum, or may be more or less calcified as in part of the suprascapula and the epiphyses of the limb bones. The bone may be cartilage bone, or membrane bone.

The skeleton is divisible into an axial portion consisting of the skull, vertebral column, and sternum, and an appendicular portion consisting of the skeleton of the limbs and their girdles.

1. The Axial Skeleton.

A. The Vertebral column.

The vertebral column is a tube, formed of a series of ten bones which surround and protect the spinal cord. Of these ten bones nine are vertebrae, while the tenth is a straight rod, the urostyle, and is almost as long as all the vertebrae put together. The second to eighth vertebrae inclusive have a very similar structure, but the first and ninth differ from the others.

Any one of the second to eighth vertebrae forms a bony ring with a somewhat thickened floor, the centrum or body, which articulates with the centra of the immediately preceding and succeeding vertebrae. The articulating surfaces are covered with cartilage and are procoelous, or convex in front and concave behind. The eighth vertebra is however amphicoelous or biconcave. The centrum of each vertebra encloses an isolated vestige of the notochord. The neural arch forms the roof and sides of the neural canal, which is very spacious in the anterior vertebrae, but becomes more depressed in the posterior ones. The arch bears the neural spine, a low median ridge of variable character, and is drawn out in front and behind, forming the two pairs of articulating surfaces or zygapophyses by means of which the vertebrae are attached together. Of these the anterior articulating surfaces or prezygapophyses look upwards and slightly inwards, while the posterior articulating surfaces or postzygapophyses look downwards and slightly outwards. The sides of the neural arches are drawn out into a pair of prominent transverse processes. Those of the second vertebra look somewhat forwards, those of the third look directly outwards or somewhat forwards, while those of the fourth, fifth, and sixth are directed slightly backwards, and those of the seventh and eighth nearly straight outwards. All the transverse processes are terminated by very small cartilaginous ribs.

Special vertebrae.

The first vertebra is a ring-like structure with a much depressed centrum. It bears in front two oval concave surfaces for articulation with the condyles of the skull, while the centrum is terminated behind by a prominent convex surface. There are as a rule no transverse processes, and the postzygapophyses look downwards and outwards. Occasionally however transverse processes do occur. Projecting forwards from the centrum is a minute process better developed in the Newt. This resembles an odontoid process, and it has hence been supposed that the first vertebra is homologous with the axis of mammalia, and that the atlas of the frog is fused with the skull.

The ninth vertebra has very stout transverse processes directed backwards and somewhat upwards. They articulate with the pelvic girdle and hence this vertebra is regarded as the sacrum. The neural arch is much depressed, the centrum is convex in front and bears on its posterior surface two short rounded processes for articulation with the urostyle.

The urostyle is a long rod-like bone forming the posterior unsegmented continuation of the vertebral column. It is probably equivalent to three vertebrae, the tenth, eleventh, and twelfth fused together, and to an unsegmented rod of cartilage which lies ventral to the notochord. The anterior end is expanded and bears two concave articular surfaces by means of which it articulates with the sacrum. A prominent ridge runs along the dorsal surface, but gradually diminishes when traced back. The anterior portion contains a canal which is a continuation of the neural canal. At a point not far from the anterior end, this canal communicates with the exterior by a pair of minute holes which correspond with the intervertebral foramina.

B. The Skull[56].

The skull of the Frog consists of three principal parts:—

(1) an axial part, the cranium proper, which encloses the brain. To it are firmly fused

(2) the capsules of the olfactory and auditory sense organs,

(3) lastly there is the hyoid apparatus and the skeleton of the jaws.

The skull is by no means so completely ossified as is the vertebral column, but in addition to the cartilage bone, there is a great development of membrane bone in connection with it.

The skull has a peculiarly flattened and expanded form depending on the wide lateral separation of the jaws from the cranium.

(1) The Cranium proper or Brain case.

This is an unsegmented tube, which is widest behind. It remains to a considerable extent cartilaginous, but is partly converted into cartilage bone, partly sheathed in membrane bone. Its roof is imperfect, being pierced by three holes or fontanelles, one large anterior fontanelle (fig. 25, A, 9), and two smaller posterior fontanelles (fig. 25, A, 10).

The cartilage bones of the cranium proper are the two exoccipitals and the sphenethmoid.

The exoccipitals (figs. 24, 25, and 26, 6) are a pair of irregular bones bounding the foramen magnum at the posterior end of the skull. They almost completely surround the foramen magnum, and bear a pair of oval convex surfaces, the occipital condyles, with which the first vertebra articulates. The bones generally called the exoccipitals of the frog include the epi-otic and opisthotic elements of many skulls, in addition to the exoccipitals.

Fig. 24. A dorsal, and B ventral views of the cranium of a Common Frog (Rana temporaria) × 2 (after Parker). In this and the next two figs. cartilage is dotted, cartilage bones are marked with dots and dashes, membrane bones are left white.

1. sphenethmoid.	14. vomer.
2. fronto-parietal.	15. posterior nares.
3. pterygoid.	16. palatine.
4. squamosal.	18. columella.
6. exoccipital.	19. quadrate.
7. parasphenoid.	20. occipital condyle.
8. pro-otic.	II. optic foramen.
9. quadratojugal.	V. VII. foramen for exit of
10. maxillae.	trigeminal and facial nerves.
11. nasal.	IX. X. foramina for exit of
12. premaxillae.	glossopharyngeal and
13. anterior nares.	pneumogastric nerves.

The patch of unossified cartilage immediately external to the occipital condyle is pierced by two small foramina, through which the ninth and tenth nerves leave the cranial cavity. The ninth nerve passes through the more external of these foramina, the tenth through the one nearer the condyle. The foramina lie however very close together and are sometimes confluent. The cranial walls for a considerable distance in front of the occipitals are unossified, but the anterior end of the cranial cavity is encircled by another cartilage bone, the sphenethmoid (figs. 24 and 25, 1) or girdle bone. This partly corresponds to the orbitosphenoids of the Newt's skull. Anteriorly it is pierced by a pair of small foramina through which the ophthalmic branches of the trigeminal nerve pass out.

The anterior part of the cranial cavity is divided into two halves by a vertical plate, the mesethmoid. Some little distance behind the sphenethmoid the ventro-lateral walls of the cartilaginous cranium are pierced by a pair of rather prominent holes, the optic foramina (figs. 24 and 25, B, II), and at a similar distance further back, occupying a kind of notch in the pro-otic are the large trigeminal foramina, through which the fifth and seventh nerves leave the cranium. Between the trigeminal and optic foramina are the very small foramina for the sixth nerves (fig. 25, B, VI).

The membrane bones of the cranium proper include the fronto-parietals and the parasphenoid.

The fronto-parietals (figs. 24 and 26, A, 2) form a pair of long flat bones closely applied to one another in the middle line, the line of junction being the sagittal suture. They cover over the fontanelles and overlap the sphenethmoid in front.

The parasphenoid (figs. 24 and 26, B, 7) is a bone shaped like a dagger with a very short handle. It lies on the ventral surface of the cranium, the blade being directed forwards and underlying the sphenethmoid; its lateral processes underlie the auditory capsules.

(2) The sense capsules.

The sense capsules are cartilaginous or bony structures which surround the olfactory and auditory organs and are closely united to the cranium.

Fig. 25. A dorsal and B ventral view of the cranium of a common Frog (Rana temporaria) from which the membrane bones have mostly been removed. × 2 (after Parker).

1. sphenethmoid.	8. pro-otic.
2. palatine.	9. anterior fontanelle.
3. pterygoid.	10. right posterior fontanelle.
4. quadrate.	11. quadratojugal.
5. columella.	12. nasal capsule.
6. exoccipital.	II. V. VI. IX. X. foramina for
7. ventral cartilaginous wall of	exit of cranial nerves.
cranium.

The auditory capsules are fused with the sides of the posterior end of the cranium just in front of the exoccipitals. They are largely cartilaginous, but include in their anterior walls a pair of irregular cartilage bones, the pro-otics (figs. 24 and 25, 8). The cartilaginous area lying ventral to the pro-otic and external to the exoccipital is pierced by a rather prominent hole, the fenestra ovalis, which forms a communication between the internal ear cavity, and a space the tympanic cavity, which lies at the side of the head, and is bounded externally by the tympanic membrane. The fenestra ovalis is occupied by a minute cartilaginous structure, the stapes, and articulated partly to this and partly to a slight recess in the pro-otic is the columella (fig. 25, B, 5), a rod in part bony and in part cartilaginous, whose outer end is attached to the tympanic membrane. The columella and stapes are together homologous with the mammalian auditory ossicles and with the hyomandibular of Elasmobranchs. Sometimes the term columella is used to include the whole ossicular chain,—the columella together with the stapes.

The olfactory or nasal capsules (fig. 25, B, 12) are fused with the anterior end of the cranium and differ from the auditory capsules in being to a great extent unossified. There are however two pairs of membrane bones developed in connection with them, the vomers and the nasals. They are drawn out into three pairs of cartilaginous processes, on the dorsal surface into the prenasal and alinasal processes which bound the external nares, and on the ventral surface towards the middle line into the forwardly-projecting rhinal processes.

The nasals (figs. 24 and 26, 11) form a pair of triangular bones lying dorsolaterally in front of the fronto-parietals. Their bases are turned towards one another and their apices are directed outwards and backwards. They correspond in position with the prefrontals of the reptilian skull as well as with the nasals.

The vomers are a pair of irregular bones lying on the ventral surface of the olfactory capsules. Each bears on its inner and posterior angle a group of minute pointed teeth, while its outer border is drawn out into three or four small slightly diverging processes, the two posterior of which form the inner boundary of the posterior nares (fig. 24, B, 15).

(3) The jaws.

The upper jaw consists of a rod of cartilage connected with the cranium near its two ends, but widely separated from it in the middle. It is almost completely overlain by membrane bone. With its posterior end the lower jaw articulates.

The membrane bones of the upper jaw include first the premaxillae, a small bone meeting its fellow in the middle line, and forming the extreme anterior end of the upper jaw. It gives off on its dorsal surface a backwardly-projecting process. It is connected behind with the maxillae (figs. 24 and 26, 10), a long flattened bone which forms the greater part of the margin of the upper jaw, and gives off near its anterior end a short process which projects upwards and meets the nasal.

Fig. 26. A, Lateral view of the skull, B, Posterior view of the cranium of a Common Frog (Rana temporaria) × 2 (after Parker).

1. sphenethmoid.	14. mento-meckelian.
2. fronto-parietal.	15. dentary.
3. pterygoid.	16. angulo-splenial.
4. squamosal.	17. basilingual plate.
5. tympanic membrane.	19. quadrate.
6. exoccipital.	20. columella.
7. parasphenoid.	21. occipital condyle.
8. pro-otic.	22. anterior cornu of the hyoid
9. quadratojugal.	(cerato-hyal).
10. maxillae.	23. foramen magnum.
11. nasal.	II. IX. X. foramina for the exit
12. premaxillae.	of cranial nerves.
13. anterior nares.

Both maxillae and premaxillae are grooved ventrally, and bear, attached to the outer more prominent margin of the groove, a row of minute conical teeth. These teeth are pleurodont, that is, are ankylosed by their bases and outer sides to the margin of the jaw. Each tooth is a hollow cone, the basal part of which is formed of bone, the apical part of dentine, capped by a very weak development of enamel.

The posterior end of the maxillae is overlapped by a small bone, the quadratojugal (figs. 24 and 26, 9), whose posterior end forms part of the articular surface for the lower jaw. Just behind the quadratojugal there is a small unossified area which lies at the angle of the mouth, and is connected by a narrow bar of cartilage with the cranium; this forms the quadrate (figs. 24 and 26, 19). A backwardly-directed outgrowth from the cartilaginous bar more or less completely surrounds the tympanic membrane, forming the tympanic ring. When followed back the maxillae and quadratojugal diverge further and further from the cranium, till the angle of the mouth comes to be separated from the foramen magnum by a space nearly double the width of the cranium. This space is bridged over to a considerable extent by two triradiate bones, the pterygoid and squamosal.

The pterygoid (figs. 24 and 26, 3) is a large bone, whose anterior limb runs forwards meeting the maxillae and palatine; while its inner limb meets the auditory capsule and parasphenoid, and its outer limb runs backwards and outwards to the angle of the mouth. The palatine is a small transversely-placed bone, which connects the pterygoid with the anterior part of the sphenethmoid. The squamosal (figs. 24 and 26, 4) is a T-shaped bone whose anterior arm is pointed and passes forwards to meet the pterygoid. The posterior upper arm is closely applied to the pro-otic, while the posterior lower arm meets the pterygoid and quadratojugal at the angle of the jaw, and surrounds the narrow cartilaginous bar of the quadrate which goes to join the cranium. The squamosal is probably homologous with the squamosal together with the pre-opercular of Bony Ganoids.

The quadrate and squamosal form the suspensorium by which the lower jaw is connected with the cranium.

The lower jaw or mandible consists of a pair of cartilaginous rods (Meckel's cartilages) in connection with each of which there are developed two membrane bones and one cartilage bone. The cartilage bone is the mento-meckelian (fig. 26, A, 14), a very small ossification at the extreme anterior end. The membrane bones are the angulo-splenial and the dentary. The angulo-splenial is a strong flat bone which forms the inner and lower part of the mandible for the greater part of its length. Its dorsal surface is produced into a slight coronoid process. The dentary (fig. 26, A, 15) is a flat plate which covers the outer surface of the anterior half of the mandible, as far forwards as the mento-meckelian. The lower jaw is devoid of teeth. The part of Meckel's cartilage which in most vertebrates ossifies, forming the articular bone, remains unossified in the Frog.

The Hyoid apparatus.

The hyoid of the adult Frog is formed of the modified hyoid and branchial arches of the tadpole. It consists of a broad thin plate of cartilage, the basilingual plate (fig. 29, B, 1), drawn out into two pairs of long processes, the cornua. The basilingual plate is broader in front than behind, and is formed from the fused ventral ends of the hyoid and branchial arches of the tadpole.

The anterior cornua (fig. 29, B, 2) form a pair of long slender cartilaginous rods which project from the body of the hyoid at first forwards, then backwards, and finally upwards and somewhat forwards again, to be united to the auditory capsules just below the fenestrae ovales. They are formed from the dorsal portion of the hyoid arch of the tadpole and are homologous with the cerato-hyals of the Dogfish.

The posterior cornua form a pair of straight bony rods diverging outwards from the posterior end of the basilingual plate. They are formed from the fourth branchial arches of the tadpole, and differ from the rest of the hyoid apparatus in being well ossified.

The columellar chain, which has been already described (p. 157), should be mentioned with the hyoid as it is homologous to the hyomandibular of fishes.

The sternum of the Frog, though regarded as part of the axial skeleton, is so intimately connected with the pectoral girdle, that it will be described with the appendicular skeleton.

2. The Appendicular Skeleton.

This consists of the skeleton of the two pairs of limbs and their respective girdles. It is at first entirely cartilaginous but the cartilage becomes later on mainly replaced by bone. The only bone developed in connection with the appendicular skeleton, which has no cartilaginous predecessor, is the clavicle.

A. The Pectoral girdle.

This consists originally of two half rings of cartilage encircling the sides of the body a short way behind the head. These two halves meet one another in the ventral middle line, and separate the anterior elements of the sternum from the posterior ones.

Each half-ring bears on the middle of its outer and posterior surface a prominent cup, the glenoid cavity, with which the proximal arm-bone articulates. This cup divides the half-arch into a dorsal scapular and a ventral coracoid portion.

The scapular portion consists of two parts, the suprascapula and the scapula.

The suprascapula (fig. 30, A, 2) is a wide, thin plate attached by its ventral and narrowest border to the scapula. Its proximal and anterior half is imperfectly ossified, its whole border or sometimes only its dorsal and posterior borders consist of unaltered hyaline cartilage, while the rest of it is composed of calcified cartilage. The scapula (fig. 30, A, 3) is a fairly stout rod of bone constricted in the middle, and forming the dorsal half of the glenoid cavity.

The coracoid portion consists of three parts, the coracoid, precoracoid and clavicle.

The largest and most posterior of these is the coracoid (fig. 30, A, 4) which like the scapula, is contracted in the middle and expanded at the ends, especially at the ventral end. It forms a large part of the glenoid cavity. The ventral ends of the coracoids which meet one another in the middle line are unossified, and form narrow strips of calcified cartilage, the epicoracoids (fig. 30, A, 5); these are often regarded as sternal elements.

The precoracoid forms a narrow strip of cartilage lying in front of the coracoid, from which it is separated by the wide coracoid foramen (fig. 30, A, 9). The dorsal end is continuous with an area of unossified cartilage which separates the coracoid and scapula and forms part of the glenoid cavity.

The clavicle is a narrow membrane bone closely attached to the anterior surface of the precoracoid, its dorsal end is expanded.

The Sternum.

The sternum consists of four parts arranged in two groups; two parts to each group. The anterior members are the episternum and omosternum.

The episternum (fig. 30, A, 10) is a thin almost circular plate of cartilage much of which remains hyaline.

The omosternum (fig. 30, A, 11) is a slender bony rod widest at its posterior end; it connects the episternum with the ventral ends of the precoracoids.

The sternum proper is a short rod of cartilage sheathed in bone; it is contracted in the middle and expanded at each end. It bears attached to its posterior end a broad somewhat bilobed plate of partially calcified cartilage, the xiphisternum (fig. 30, A, 13).

B. The Anterior limb.

This is divisible into three parts, the upper arm or brachium, the fore-arm or antibrachium, and the manus.

All the larger bones have their ends formed by prominent epiphyses which do not unite with the shaft till late in life. Their articulating surfaces are covered by hyaline cartilage.

In the upper arm there is a single bone, the humerus.

This has a more or less cylindrical shaft and articulates by a prominent rounded head with the glenoid cavity. The distal end shows a large rounded swelling on either side of which is a condylar ridge, the inner or postaxial one being the larger. A prominent deltoid ridge runs along the proximal half of the anterior surface, and in the male frog a second equally prominent ridge runs along the distal half of the posterior surface.

The fore-arm consists of two bones, the radius and ulna, united together and forming the radio-ulna. The two bones are quite fused at their proximal ends where they form a deep cup which articulates with the distal end of the humerus, and is drawn out into a rather prominent backwardly-projecting olecranon process, which ossifies from a centre distinct from that of the shaft. The distal end is distinctly divided by a groove into an anterior radial and a posterior ulnar portion.

The manus consists of two parts, the wrist or carpus and the hand.

The carpus[57] consists of six small bones arranged in two rows. The three bones of the proximal row are the ulnare, radiale and centrale. The ulnare and radiale are about equal in size and articulate regularly with the radio-ulna. The centrale is pushed out of its normal position and lies partly on the pre-axial side, partly in front of the radiale. Of the three bones of the distal row the two pre-axial ones, carpalia 1 and 2, are small; carpale 2 articulates with the second metacarpal, carpale 1 with both the first and second. The third bone is large and articulates with the third, fourth and fifth metacarpals, it represents carpalia 3-5, with probably in addition the representative of a second centrale.

The hand consists of four complete digits, and a vestigial pollex reduced to a short metacarpal.

Each of the four complete digits consists of a metacarpal and a variable number of phalanges. The first digit, as just mentioned, has no phalanges, the second and third have two, and the fourth and fifth have three.

C. The Pelvic girdle.

The pelvic girdle of the Frog is much modified from the simple or general type found in the Newt (p. 149).

It is a V-shaped structure consisting of two halves which are fused together in the middle line posteriorly, while in front they are attached to the ends of the transverse processes of the sacral vertebra. Each half bears at its posterior end a deep cup, the acetabulum, with which the head of the femur articulates.

Each half of the pelvis ossifies from two centres. The anterior and upper half of the acetabulum, and the long laterally compressed bar extending forwards to meet the sacral vertebra ossify from a single centre and are generally called the ilium; it is probable however that they represent both the ilium and pubis of mammals[58]. The posterior part of this bone meets its fellow in a median symphysis.

The posterior third of the acetabulum is formed by a small bone, the ischium, which likewise meets its fellow in a median symphysis.

The ventral portion of the pelvic girdle never ossifies, even in old animals being formed only of calcified cartilage. This is generally regarded as the pubis, but it perhaps corresponds to the acetabular bone of mammals.

D. The Posterior limb.

This corresponds closely to the anterior limb and, like it, is divisible into three parts, the thigh, the shin or crus and the pes.

As was the case with the anterior limb, all the long bones have their ends formed by prominent epiphyses which do not unite with the shaft till late in life.

In the thigh there is only a single bone, the femur.

The femur is a moderately long, slender bone with a well-ossified hollow shaft slightly curved in a sigmoid manner. Both ends are expanded, the proximal end is hemispherical and articulates with the acetabulum, the distal end is larger and more laterally expanded.

The shin likewise includes a single bone, the tibio-fibula, but this, as can be readily seen by the grooves at the proximal and distal ends of the shaft, is formed by the fusion of two distinct bones, the tibia and fibula. The tibio-fibula is longer and straighter than the femur.

The pes consists of two parts, the ankle or tarsus and the foot.

The tarsus consists of two rows of structures, very different in size. The proximal row consists of two long bones, the tibiale and fibulare, which are united by common epiphyses at the two ends, while in the middle they are widely separated. The tibiale lies on the tibial or pre-axial side, and the fibulare which is the larger of the two bones on the fibular or postaxial side. The distal row of tarsals consists of three very small pieces of calcified cartilage. The postaxial of these is the largest, it articulates with the second and third metatarsals and is probably homologous with tarsalia 2 and 3 fused. The middle one is very small, it articulates with the first metatarsal and is probably tarsale 1. The pre-axial one articulates with the metatarsal of the calcar, a structure to be described immediately, and has been regarded as a centrale.

The foot includes five complete digits and a supplemental toe as well. Each of the five digits consists of a long metatarsal with epiphyses at both ends, and of a variable number of phalanges. The first digit or hallux and the second have two phalanges, the third three, the fourth, which is the largest, four, and the fifth, three. The distal phalanges have epiphyses only at their proximal ends, the others at both ends.

On the pre-axial side of the hallux is the supplemental digit, the prehallux or calcar. It consists of a short metatarsal and one or two phalanges, and is terminated distally by a horny covering of epidermal origin.

EXOSKELETON.

The exoskeleton, at any rate in most living forms, is very slightly developed in Amphibia. The only representatives of the epidermal exoskeleton are (1) the minute horny beaks found coating the premaxillae and dentaries in Siren and the tadpoles of most Anura, (2) the nails borne by the first three digits of the pes in Xenopus and by the Japanese Salamander Onychodactylus, (3) the horny covering of the calcar or prehallux of frogs. The Urodela and nearly all the Anura, which form the vast majority of living Amphibia, have naked skins. A few Anura belonging to the genera Ceratophrys and Brachycephalus have bony dermal plates developed in the skin of the back, and these plates become united with some of the underlying vertebrae.

In the Gymnophiona the integument bears small cycloid scales arranged in rings which are equal in number to the vertebrae. These scales contain calcareous concretions. Scales also occur between the successive rings.

In the Labyrinthodontia the dermal exoskeleton is in many genera greatly developed. It is generally limited to the ventral surface and consists principally of a buckler formed of three bony plates, one median and two lateral. These plates protect the anterior part of the thorax, and are closely connected with the adjacent endoskeleton. They probably represent the interclavicle and clavicles. Behind this buckler numerous scutes are generally developed, which often cover the whole ventral surface, and may cover the whole body.

Teeth[59].

In Amphibia teeth are generally present on the maxillae, premaxillae and vomers, and except in Anura on the dentaries; sometimes they occur on the palatines as in many Urodela, most Labyrinthodontia, and the Gymnophiona; less commonly on the pterygoids as in Menobranchus, Siredon, some Labyrinthodontia, and Pelobates cultripes[60], or on the splenials as in Siren and Menobranchus, or parasphenoid as in Pelobates cultripes, Spelerpes belli and Batrachoseps. In some Anura such as Bufo and Pipa the jaws are toothless.

In Gymnophiona, Menobranchus, and Siredon, the teeth are arranged in two concentric curved rows. The teeth of the outer row are borne on the premaxillae and maxillae if present, (the maxillae are absent in Menobranchus), the teeth of the second row on the vomers and pterygoids in Menobranchus and Siredon, and on the vomers and palatines in Gymnophiona. In some Gymnophiona there is a double row of mandibular teeth. The vomerine, palatine and parasphenoid teeth of all forms are numerous and are not arranged in rows.

The teeth of all living Amphibia are simple conical structures ankylosed to the bone, and consisting of dentine, coated or capped with a thin layer of enamel. In the Labyrinthodontia teeth of more than one size are sometimes present. The dentine of the basal part of the larger teeth is in some genera very greatly folded, causing the structure to be highly complicated. These folds, the intervals between which are filled with cement, radiate inwards from the exterior and outwards from the large pulp cavity. The basal part of the teeth of Ceratophrys (Anura) has a similar structure.

ENDOSKELETON.

Vertebral column.

Four regions of the vertebral column can generally be recognised in Amphibia, viz. the cervical, the trunk or thoraco-lumbar, the sacral and the caudal regions. In the limbless Gymnophiona, however, only three regions, the cervical, thoracic, and post-thoracic can be made out. The cervical region is limited to a single vertebra which generally differs from the others in having no transverse processes or indication of ribs. It is generally called the atlas, but it commonly bears a small process arising from the anterior face of the centrum which resembles the odontoid process of higher animals, and renders it probable that the first vertebra of Amphibia corresponds to the axis, not to the atlas. Amphibia generally have a single sacral vertebra.

Three elements go to make up the vertebral column in Amphibia, viz.

1. the notochord,

2. the long vertebral centra,

3. intervertebral cartilage which forms the joints between successive centra.

The relations which these three elements bear to one another are subject to much variation. The successive stages can be well traced in the Urodela.

1. The first stage is found in larval Urodeles in general and in adult Ichthyoidea, and some Salamandrina. In these forms the notochord persists and retains approximately the same diameter throughout the whole length of the vertebral column. Bony biconcave centra are present and constrict it to a certain extent vertebrally, while intervertebrally there is a development of cartilage. The connection between the bony vertebrae is effected mainly by the expanded notochord.

2. In the next stage, as seen in Gyrinophilus porphyriticus, the growth of intervertebral cartilage has caused the almost complete obliteration of the notochord intervertebrally, and its entire disappearance vertebrally, i.e. in the centre of each vertebra. The intervertebral cartilage now forms the main connection between successive vertebrae, and sometimes cases are found whose condition approaches that of definite articulations. Readily recognisable remains of the notochord are still found at each end of the intervertebral constriction.

3. In the third stage differentiation and absorption of the intervertebral cartilage has given rise to definitely articulating opisthocoelous vertebrae. These are found in most adult Salamandrina.

The transverse processes of the earlier trunk vertebrae are divided into two parts, a dorsal part which meets the tubercular process of the rib and is derived from the neural arch, and a ventral part which meets the capitular process of the rib, and is derived from the ventral or haemal arch. In the caudal vertebrae and often also in the posterior trunk vertebrae the two processes are fused.

Siren and Proteus, although they possess minute posterior limbs, have no sacral vertebrae, while Cryptobranchus lateralis has two. The caudal vertebrae, except the first, have haemal arches very similar to the neural arches.

In Labyrinthodontia the centra of the vertebrae are generally well ossified biconcave discs. In some forms however, like Euchirosaurus, the centra are imperfectly ossified, and consist of bony rings traversed by a wide notochordal canal. Each ring is formed of four pieces, a large well-ossified neural arch, a basal piece, and a pair of lateral pieces. Vertebrae of this type are called rachitomous.

In the tail region of other forms each vertebra consists of an anterior centrum bearing the neural arch, and a posterior intercentrum[61] bearing chevron bones. Vertebrae of this type are called embolomerous. Haemal arches similar to the neural arches are often found as in Urodela. The transverse processes are sometimes well developed and are divided into tubercular and capitular portions.

In Gymnophiona the vertebrae are biconcave and are very numerous, they sometimes number about two hundred and thirty. Only quite the last few are ribless and so can be regarded as post-thoracic vertebrae. The first vertebra has nothing of the nature of an odontoid process.

In Anura the number of vertebrae is very greatly reduced, only nine and the urostyle being present. Of these, eight are presacral and one sacral. The urostyle is post-sacral and corresponds to three or more modified vertebrae. The first vertebra is without transverse processes, the remaining presacral vertebrae have the transverse processes fairly large, while the sacral vertebra has them very large, forming in some genera widely expanded plates. The urostyle is a long cylindrical rod which articulates with the sacrum generally by two facets. Ankylosed to its anterior end are the remains of two neural arches.

In Anura remains of the notochord are found in the centre of each vertebra, i.e. vertebrally, while in the Urodela they only occur intervertebrally.

The vertebrae in Anura are, as a rule, procoelous. The eighth vertebra is however generally amphicoelous, while the ninth commonly has one convexity in front, and two behind.

In some forms such as Bombinator, Pipa, Discoglossus and Alytes they are opisthocoelous; in others like Pelobates they are variable.

The Skull[62].

Cranium and mandible.

In the Amphibian skull there are as a rule far fewer bones than in the skull of bony fish. The primordial cartilaginous cranium often persists to a great extent. Only quite a few ossifications take place in it; namely in the occipital region—the exoccipitals, further forwards—the pro-otics, and at the boundary of the orbital and ethmoidal regions—the sphenethmoid. The basi-occipital and basisphenoid are never ossified. As in Mammalia there are two occipital condyles formed by the exoccipitals.

Large vacuities commonly occur in the cartilage of both floor and roof of the primordial cranium. These are roofed over to a greater or less extent by the development of membrane bone. Thus on the roof of the cranium there are paired parietals, frontals, and nasals, and on its floor are paired vomers, and a median unpaired parasphenoid.

In all living forms the parietals meet and there is no interparietal foramen, though this exists in Labyrinthodonts.

The palato-pterygo-quadrate bar is united at each end with the cranium, but elsewhere in most cases forms a wide arch standing away from it. The suspensorium is, as in Dipnoi and Holocephali, autostylic. The palato-pterygo-quadrate bar sometimes remains entirely cartilaginous, sometimes its posterior half is ossified forming the quadrate. In connection with it a number of membrane bones are generally developed, viz. the maxillae, premaxillae, palatines, pterygoids, quadratojugals, and squamosals. The pterygoids are, however, sometimes partially formed by the ossification of cartilage. The cartilage of the lower jaw and its investing membrane bones generally have much the same relations as in bony fishes.

Urodela. The skulls of the various Urodeles show an interesting series of modifications and differ much from one another, but all agree in the absence of the quadratojugals, in the fact that the palatines lie parallel to the axis of the cranium, and in the large size of the parasphenoid.

The lower types Menobranchus, Siren, Proteus, and Amphiuma have longer and narrower skulls than do the higher types.

Menobranchus has a very low type of skull which remains throughout life in much the same condition as that of a young tadpole or larval salamander. The roof and floor of the cranium internal to the membrane bones are formed of fibrous tissue, not of well-developed cartilage. The epi-otic regions of the skull are ossified, forming a pair of large bones which lie external to, and distinct from, the exoccipitals. Proteus and the Labyrinthodonts are the only other Amphibia which have these elements separately ossified. The parietals send a pair of long processes forwards along the sides of the frontals. Nasals and maxillae are absent, as is likewise the case in Proteus. Teeth are borne on the vomers, premaxillae, pterygoids, dentaries and angulo-splenials. The suspensorium is forwardly directed.

The skull of Siren resembles that of Menobranchus in several respects, as in the forward direction of the suspensorium and in the absence of maxillae, but differs in the possession of nasals, in the toothless condition of the premaxillae and dentaries, and in the fusion and dentigerous condition of the vomers and palatines.

Amphiuma has a skull which, though narrow and elongated, differs from those of Menobranchus, Proteus, and Siren, and resembles those of higher types in the following respects:—

(1) the suspensorium projects nearly at right angles to the cranium instead of being directed forwards, (2) the maxillae are well developed, and the premaxillae are completely ankylosed together, (3) there are no palatines.

The skulls of Megalobatrachus, Cryptobranchus and Siredon resemble those of the highest Urodeles the Salamanders in their wide form, in having the pro-otics distinct from the exoccipitals which are ossified continuously with the epi-otics and opisthotics, and in having no palatines, but differ in having the two premaxillae separate, and in the arrangement of the vomerine teeth which in Megalobatrachus and Cryptobranchus are placed along the anterior boundaries of the bones, these meeting in the middle line. In Siredon the vomers are separated by the very large parasphenoid.

The suspensorium in Megalobatrachus and Cryptobranchus projects at right angles to the cranium; in Siredon it projects somewhat downwards and forwards as in the Salamandrina.

Modifications of the vomers, pterygoids and palatines accompany the changes of the larval ichthyoid Siredon into the adult salamandroid Amblystoma, the vomers especially come to resemble to a much greater extent those of the Salamandrina.

The ossification of the skull in the Salamandrina is carried further than in the Ichthyoidea, though the supra-occipital and basi-occipital are not ossified. The skull differs from that in the Ichthyoidea in the size of the part of the vomero-palatines which lies in front of the teeth, in the frequent union of the two premaxillae and in the ossification of all the periotic bones continuously with the exoccipital.

The skull differs from that of Anura in the following respects:—

(1) the bones of the upper jaw do not form a complete arch standing away from the cranium, and the maxillae are not united to the quadrates by quadratojugals, (2) the long axis of the suspensorium passes obliquely downwards and forwards instead of downwards and backwards, (3) there is no sphenethmoid encircling the anterior end of the brain, its place being partly taken by a pair of orbitosphenoids, (4) there is no definite tympanic cavity.

Fig. 27. Dorsal view of the skull of a Labyrinthodont (Capitosaurus nasutus) × 1/9 (from von Zittel).

1. premaxillae.	12. postorbital.
2. nasal.	13. interparietal foramen.
3. maxillae.	14. squamosal.
4. anterior nares.	15. supratemporal.
5. frontal.	16. quadratojugal.
6. prefrontal.	17. quadrate.
7. lachrymal.	18. epi-otic.
8. jugal.	19. dermo-supra-occipital.
9. orbit.	20. exoccipital.
10. parietal.	21. foramen magnum.
11. postfrontal.

Labyrinthodontia. The skull in Labyrinthodontia is remarkable for its extreme solidity, the large number of bones which are present, and the extent to which the roofing over of the temporal and other fossae has taken place. In many forms the surface of the bones is as in Crocodiles, strongly sculptured (fig. 27, right half) with ridges and grooves which probably lodged sensory organs. The bones forming the roof of the skull are generally very uniform in size, perhaps the most noticeable of them being the paired dermo-supra-occipitals (fig. 27, 19). Paired dermo-supra-occipitals occur also in certain Ganoids. The Labyrinthodont skull also bears resemblance to that of many fish in the development of a pair of long pointed epi-otics (fig. 27, 18), which remain permanently distinct from the surrounding bones. The parietals are small and enclose between them the interparietal foramen (fig. 27, 13). In some forms in which the head is protected with an armour of scutes, these do not roof over the interparietal foramen, and from this fact it has been inferred that the Labyrinthodonts had a functional pineal eye. Both supra- and infra-temporal fossae are partially or completely roofed over by the postorbitals and large supra-temporals (fig. 27, 15).

There is generally a ring of bones in the sclerotic coat of the eye. The pterygoids do not meet in the middle line, being separated by the parasphenoid. The palatines bear teeth, and in some genera (Archegosaurus) form long splints lying along the inner side of the maxillae and more or less surrounding the posterior nares. In others (Nyrania) they lie in the normal position near the middle line, one on each side of the parasphenoid. The vomers bear teeth and sometimes meet in the middle line; they are sometimes confluent with the parasphenoid. On the ventral surface of the cranium there are generally large palatal vacuities.

In the mandible there is often a well-marked postglenoid process, and the articular is generally completely ossified.

Fig. 28. A, ventral view of the cranium; B, lateral view of the cranium and mandible of Siphonops annulatus (after Wiedersheim).

1. anterior nares.	9. squamosal.
2. naso-premaxillae.	10. exoccipital.
3. frontal.	11. dentary.
4. parietal.	12. angular.
5. maxillae.	13. basi-occipital and
6. vomer.	basisphenoid fused.
7. orbit.	14. posterior narial opening
8. quadrate united with the	surrounded by the palatine.
pterygoid in front.	X. pneumogastric foramen.

Gymnophiona. The skull bears a considerable resemblance to that of Labyrinthodonts, especially in the arrangement of the bones which bound the mouth cavity. The cranium is very hard, and is covered by a complete bony roof formed mainly of the exoccipitals, parietals, frontals, prefrontals, nasals and premaxillae. The nasals and premaxillae are sometimes ossified continuously. There is a median unpaired ethmoid whose dorsal end appears at the surface wedged in between the frontals and parietals. The bone generally regarded as the squamosal[63] is very large, and it and the maxillae generally together surround the orbit, which, in Epicrium, has in it a ring of bones. The palatines form long tooth-bearing bones fused with the inner sides of the maxillae; they nearly surround the posterior nares.

The quadrate bears the knob, and the angular the cup for the articulation of the mandible,—a very primitive feature. The mandible is also noticeable for the enormous backward projection of the angular.

Anura. In Anura the skull is very short and wide owing to the transverse position of the suspensorium. There is often a small ossification representing the quadrate. Sometimes as in Hyla and Alytes there is a fronto-parietal fontanelle.

As compared with the skull in Urodela the chief characteristics of the skull of Anura are:—

1. the presence of a sphenethmoid,

2. the union of the frontals and parietals on each side,

3. the occasional occurrence of small supra- and basi-occipitals,

4. the backward growth of the maxillae and its connection with the suspensorium by means of the quadratojugal,

5. the dagger-like shape of the parasphenoid,

6. the occurrence of a definite tympanic cavity,

7. the frequent occurrence of a predentary or mento-meckelian ossification in the mandible.

The skull of Pipa is abnormal, being greatly flattened and containing little cartilage. The fronto-parietals are fused, and there is no sphenethmoid. The quadrates are well developed and the squamosals and parasphenoid differ much from those of other Anura.

Hyoid and branchial arches.

In larval Amphibia the hyoid and four branchial arches are generally present, and in adult Ichthyoidea they are frequently almost as well represented as in the larva, and are of use in strengthening the swallowing apparatus. They are very well seen in Siredon, and consist of a hyoid attached by ligaments to the suspensorium, followed by four branchial arches of which the first and second are united by a copula (fig. 29, D, 8), while the third and fourth are not. The hyoid is not always the largest and best preserved of the arches, for sometimes as in Spelerpes one of the branchials is far larger than the hyoid. Four branchial arches occur in Siren as in Siredon, but in Proteus there are only three.

In some larval Labyrinthodontia (Branchiosaurus) four branchial arches are known to occur, and their arrangement is almost precisely similar to that in Siredon.

In Gymnophiona the remains of only three branchial arches occur in addition to the hyoid. The four arches are all very similar to one another, each consists of a curved rod of uniform diameter throughout. The hyoid is united with the first branchial arch, but has no attachment to the cranium.

In larval Anura (fig. 29, C) the arrangement of the hyoid and branchial arches is much as in Urodela. In the adult, however, the ventral parts of all the arches unite, forming a compact structure, the basilingual plate (fig. 29, B, 1).

Fig. 29. Visceral arches of Amphibia.

A. Molge cristata	(after Parker).
B. Rana temporaria	adult (after Parker).
C. Tadpole of Rana	(after Martin St Ange).
D. Siredon pisciformis	(after Credner).

In each case the ossified portions are slightly shaded, while the cartilaginous portions are left white.

1. basilingual plate.	5. third branchial arch.
2. hyoid arch.	6. fourth    do.
3. first branchial arch.	7. thyro-hyal.
4. second    do.	8. copula.

The dorsal parts of the first three branchial arches disappear, but those of the fourth become ossified and form the short, stout thyro-hyals or posterior cornua. The dorsal parts of the hyoid arch in the adult form a pair of long bars, the anterior cornua, which are united to the periotic region of the skull in front of the fenestra ovalis either by short ligaments or by fusion as in Bufo. In Pipa and Xenopus the first and second branchial arches persist as well as the fourth (thyro-hyal), but in Pipa the hyoid is wanting.

Ribs.

Ribs are generally very poorly developed in Amphibia. In Anura they are in most cases absent; when present they generally form minute unossified appendages attached to the transverse processes, but in Discoglossus and Xenopus the anterior vertebrae are provided with distinct ribs. In Urodela and Labyrinthodontia they are generally short structures, each as a rule attached to the vertebra by a bifurcated proximal end. The number of rib-bearing vertebrae varies, but the first and the posterior caudal vertebrae are always ribless. The anterior caudal vertebrae too are generally ribless, but sometimes a few of them bear small ribs. In Spelerpes the last two trunk vertebrae are ribless, and hence may be regarded as lumbar vertebrae.

In Gymnophiona ribs are better developed than in any other Amphibia; they occur on all the vertebrae except the first and last few, and are attached to the transverse processes, sometimes by single, sometimes by double heads.

Sternal ribs are almost unknown in Amphibia, but traces of them occur in Menobranchus.

Sternum.

In Amphibia the sternum is not very well developed; sometimes as in Gymnophiona and Proteus no traces of it occur, and in the Urodela it is never ossified. It is always very intimately related to the pectoral girdle. In the Salamandrina it has the form of a broad thin plate of cartilage, grooved and overlapped by the coracoid.

Fig. 30. Shoulder-girdle and sternum of
A. An old male common Frog (Rana temporaria).
B. An adult female Docidophryne gigantea (after Parker).

In both A and B the left suprascapula is removed. The parts left
unshaded are ossified; those marked with small dots consist of hyaline
cartilage, those marked with large dots of calcified cartilage.

1. calcified cartilage of	7. clavicle.
suprascapula.	8. glenoid cavity.
2. ossified portion of	9. coracoid foramen.
suprascapula.	10. episternum.
3. scapula.	11. omosternum.
4. coracoid.	12. sternum.
5. epicoracoid.	13. xiphisternum.
6. precoracoid.

In most Anura the sternum consists of a number of parts arranged in series. At the anterior end is a flat cartilaginous plate with a bony basal stalk. This plate is called the episternum, and its stalk the omosternum. The continuity of the sternum is now interrupted by a pair of cartilaginous structures, the epicoracoids, which are shoulder-girdle elements, and represent the unossified ventral ends of the coracoids. In some cases cartilaginous epiprecoracoids can also be distinguished. Further back is the long sternum proper, while last comes the xiphisternum, a broad expanded plate of cartilage.

In some Anura such as Pipa and Hyla the number of sternal elements is considerably reduced.

Appendicular Skeleton.

Pectoral girdle.

The most primitive Amphibian shoulder-girdle is found in the Urodela. It consists of a dorsal element, the scapula, a posterior ventral element, the coracoid, and an anterior ventral element, the precoracoid. The clavicle is not developed, and the two coracoids overlap in the middle line. The shoulder-girdle remains largely cartilaginous but the proximal end of the scapula is ossified, and the ossification may extend through part of the coracoid and precoracoid.

In Labyrinthodontia there is an exoskeletal ventral buckler formed of three plates, a median one, which probably represents an interclavicle, and two lateral ones, which are probably clavicles. Traces of endoskeletal structures, probably corresponding to the precoracoid and scapula, are also known in some cases. The Gymnophiona and some of the Labyrinthodontia have lost the pectoral girdle and limbs.

The ossification of the shoulder-girdle has gone on much further in Anura than it has in Urodela. Clavicles are present and the scapula and coracoid of each side are ossified from separate centres. The distal part of the scapula forms a large imperfectly ossified plate, the suprascapula.

The shoulder-girdle of Anura is however subject to considerable variations. In the Toads (Bufonidae) the epicoracoids or unossified ventral ends of the coracoids and precoracoids overlap in the middle line (fig. 30, B, 5). This arrangement is called Arciferous. In the Frogs,—Ranidae, and other forms belonging to the group Firmisternia,—the epicoracoids do not overlap but form a narrow cartilaginous bar separating the ventral ends of the coracoids (fig. 30, A, 5).

Anterior limb.

In many Amphibia and especially in the Urodela the anterior limb has a very simple unmodified arrangement. The humerus is straight and of moderate length, its ends are rounded for articulation on the one hand with the shoulder-girdle, and on the other hand with the radius and ulna. In the Urodela the radius and ulna are distinct. In the Anura they have fused, though the line of junction of the two is not obliterated. Their proximal ends are hollowed for articulation with the convex end of the humerus.

The manus in all recent Amphibia agrees in never having more than four complete digits, but is subject to considerable variation, this statement applying especially to the carpus.

In the larva of Salamandra (fig. 31, A), except that the pollex is absent[64], the manus retains completely the condition which is generally regarded as primitive for the higher Vertebrata. It consists of an anterior row of three elements, the ulnare, intermedium, and radiale, and a posterior row of four, the carpalia 2, 3, 4, and 5. Interposed between the two rows is a centrale. Menobranchus has a similar very simple carpus. In most other Amphibia this simplicity is lost. This loss may be due to:—

(a) fusion of certain structures, e.g. in the adult Salamandra the intermedium and ulnare have fused,

(b) displacement of structures, e.g. in Bufo viridis, the centrale has been pushed up till it comes to articulate with the radius,

(c) the development of supernumerary elements, especially of extra centralia. In Megalobatrachus two or even three centralia sometimes occur.

Fig. 31. A, Right Antibrachium and Manus of a larval Salamander (Salamandra maculosa) (after Gegenbaur).
B, Right Tarsus and adjoining bones of Molge sp. (after Gegenbaur).

1. radius.	11. tibia.
2. ulna.	12. fibula.
3. radiale.	13. tibiale.
4. intermedium.	14. intermedium.
5. ulnare.	15. fibulare.
6. centrale.	16. centrale.
7. carpale 2.	17. tarsale 1.
8.  "    3.	18. tarsalia 4 and 5 fused.
9.  "    4.	I. II. III. IV. V. digits.
10.   "   5.

In the great majority of Amphibia while one digit, probably the first, is absent, the other four digits are well developed. In the forms however with degenerate limbs like Amphiuma, Siren and Proteus the number of digits is still further reduced. In Siren there are three or four, in Proteus three, and in Amphiuma two or three digits in the manus.

In Anura the pollex is represented only by a short metacarpal. There are sometimes traces of a prepollex. The carpus often has two centralia and the intermedium is absent.

In Labyrinthodontia the limbs are generally very simple and resemble those of Urodela. In some forms, however, the manus differs from that of all living Amphibia in possessing five well-developed digits.

Pelvic girdle.

The simplest Amphibian pelvis is that of some of the Labyrinthodontia; thus in Mastodonsaurus it consists dorsally of a short broad ilium placed vertically and attached to the sacrum, and ventrally of a small pubis and of a large ischium meeting its fellow in the middle line. In some Labyrinthodonts the pubes as well as the ischia meet in a ventral symphysis, and in many there are no obturator foramina. In Siren, Gymnophiona and some Labyrinthodontia the pelvic girdle and limbs are absent.

In Urodela the ventral element of the pelvis on each side forms a flat plate which meets its fellow of the opposite side. The anterior part of the plate, representing the pubis, generally remains cartilaginous throughout life; the posterior part representing the ischium is in almost every case well ossified. Attached to the anterior end of the pubes there is an unpaired bifid cartilaginous structure, the epipubis. The ilia are vertically placed.

In most Anura the pelvis is peculiarly modified in correlation with the habits of jumping. The long bone generally called the ilium is placed horizontally and is attached at its extreme anterior end to the sacrum. The ischium is ossified and distinct. Ventrally in front of the ischium there is a tract of unossified cartilage which is often regarded as the pubis. In Xenopus, however, the bone corresponding to the ilium of the Frog is seen to ossify from two centres, one forming the ilium, the other, which lies at the symphysis, being apparently the pubis. This makes it probable that the so-called ilium of the Frog is really to be regarded as an ilio-pubis, and renders the homology of the cartilaginous part uncertain, but it probably corresponds to the acetabular bone of mammals. In Xenopus also there is a minute epipubis similar to that of Urodeles.

Posterior limb.

In Urodela the posterior limb (fig. 31, B) closely resembles the anterior limb, but is even less removed from the primitive condition of the higher vertebrates in the fact that all five digits are commonly present. The tibia and fibula are short bones approximately equal in size. In some cases the number of digits is reduced. Thus in Menobranchus the pes has four digits, in Proteus it has two, and in Amphiuma two or three, while in Siren the posterior limbs have atrophied.

In correlation with their habits of jumping, the posterior limbs in Anura are much lengthened and considerably modified. The tibia and fibula are completely fused. The intermedium is absent, while the tibiale and fibulare are greatly elongated. Tarsalia 4 and 5 are absent. Five digits are always present, and there is a prehallux formed of two or more segments.

In general the posterior limbs in Labyrinthodontia bear the closest resemblance to the anterior limbs; in some cases three centralia are found.

In Ichthyoidea, and in most Labyrinthodontia, the cartilages of the carpus and tarsus remain unossified; in Salamandrina and in Anura they are generally ossified.

This great group includes the Reptiles and Birds and forms the second of the three into which the Gnathostomata may be divided. There is nearly always a strongly-developed epiblastic exoskeleton which has the form of scales or feathers, and in some cases a dermal exoskeleton is also well developed. In living forms the notochord never persists, being replaced by vertebrae, but in some extinct forms the centra are notochordal. The vertebral centra are ossified, and only in exceptionally rare cases have terminal epiphyses. The skull is well ossified and has membrane bones incorporated in its walls.

The occipital segment is completely ossified, and an interorbital septum or bony partition separating the two orbits is usually developed to a greater or less extent. The skull generally articulates with the vertebral column by a single occipital condyle into the composition of which the exoccipitals and basi-occipital enter in varying proportions. The pro-otic ossifies, and either remains distinct from the epi-otic[65] and opisthotic throughout life, or unites with them only after they have fused with the adjacent bones. The hyoid and branchial arches are much reduced; and the representative of the hyomandibular is connected with the auditory apparatus, forming the auditory ossicles[66]. Each ramus of the mandible always consists of a cartilage bone, the articular, and several membrane bones. The mandible articulates with the cranium by means of a quadrate. The ribs in Birds and some Reptiles bear uncinate processes, i.e. small, flat, bony or cartilaginous plates projecting backwards from their posterior borders. The sternum is not transversely segmented as in mammals, and there are commonly distinct cervical ribs. The ankle joint is intertarsal, or situated between the proximal and distal row of tarsal bones, not cruro-tarsal as in Mammalia.

Class I. Reptilia[67].

The axial skeleton is generally long, and that of the limbs frequently comparatively short, or sometimes absent.

The exoskeleton generally has the form of epidermal scales, which are often combined with underlying bony dermal plates or scutes and may sometimes form a continuous armour. Neither feathers nor true hairs are ever present. The vertebral column is generally divisible into the five usual regions. The centra of the vertebrae vary enormously, and may be amphicoelous, procoelous, opisthocoelous or flat, but they never have saddle-shaped articulating surfaces. The quadrate is always large, and is sometimes fixed, sometimes movable. A transpalatine bone uniting the pterygoid and maxillae is generally present.

Free ribs are often borne along almost the whole length of the trunk and tail, and often occur attached to the cervical vertebrae. The sacrum is generally composed of two vertebrae which are united with the ilia by means of expanded ribs. The sternum is rhomboidal, and may either be cartilaginous or formed of cartilage bone, but never of membrane bone; it differs from that of birds also in the fact that it does not ossify from two or more centres. An interclavicle is generally present. There are always more than three digits in the manus, and never less than three in the pes. In all living reptiles the ilia are prolonged further behind the acetabula than in front of them, and the bones of the pelvis remain as a rule, distinct from one another throughout life.

The pubes (pre-pubes) and ischia both commonly meet in ventral symphysis, and the acetabula are wholly or almost wholly ossified. The metatarsals are not ankylosed together.

Order 1. Theromorpha[68].

This order includes a number of mainly terrestrial, extinct reptiles, which differ much from one another, and show remarkable points of affinity on the one hand with the Labyrinthodont Amphibia, and on the other with the Mammalia. The vertebrae are nearly always amphicoelous and sometimes have notochordal centra. The skull is short and has the quadrate immovably fixed. There is an interparietal foramen, and generally large supratemporal fossae bounded by supratemporal arcades, but with no infratemporal[69] arcades; Elginia however has the whole of the temporal region completely roofed over.

The teeth are placed in distinct sockets and are very variable in form, the dentition sometimes resembling the heterodont dentition of mammals. The humerus has distinct condyles and an ent-epicondylar foramen[70] as in many mammals.

The pubis is fused with the ischium, and both pectoral and pelvic girdles are remarkably solid. The obturator foramen is remarkably small or even absent. The anterior ribs have two articulating surfaces, and each articulates by its tuberculum with the transverse process, and by its capitulum with the centrum as in mammals.

These reptiles occur chiefly in deposits of Triassic and Permian age. Some of the best known genera are Dicynodon, Udenodon, Placodus, Pariasaurus and Galesaurus. They will be noticed in the general account of the skeleton in reptiles.

Order 2. Sauropterygia.

This order includes a number of extinct marine reptiles, devoid of an exoskeleton. The tail is short, the trunk long, and the neck in the most typical forms extremely long. The vertebrae have slightly biconcave, or nearly flat centra. The skull is relatively small and has large supratemporal fossae. The teeth are placed in distinct sockets, and are generally confined to the margins of the jaws; they are sharp and curved and are coated with grooved enamel. The premaxillae are large, and there is an interparietal foramen. The quadrate is firmly united to the cranium. The anterior nares are separate and are placed somewhat close to the orbits. There is no ossified sclerotic ring. The palatines and pterygoids meet the vomers, and more or less completely close the palate, and in some forms, e.g. Plesiosaurus, there is a distinct parasphenoid. Thoracic ribs are strongly developed and each articulates with its vertebra by a single head. The cervical vertebrae have well-marked ribs, which articulate only with the centra, in this respect differing from those of Crocodiles. The caudal vertebrae bear both ribs and chevron bones, and abdominal splint-ribs are largely developed.

In the shoulder-girdle the coracoids are large and meet in a ventral symphysis; precoracoids and a sternum are apparently absent, but parts generally regarded[71] as the clavicles and interclavicle are well developed. In the pelvis, the pubes and ischia meet in a long symphysis. The limbs are pentedactylate, and in the best known forms, the Plesiosauridae, form swimming paddles.

The Sauropterygia occur in beds of Secondary age, and some of the best known genera are Plesiosaurus, Pliosaurus and Nothosaurus.

Order 3. Chelonia.

In the Tortoises and Turtles the body is enclosed in a bony box, formed of the dorsal carapace, and a flat ventral buckler, the plastron. Except in Dermochelys the carapace is partly formed from the vertebral column and ribs, partly from dermal bones. Both carapace and plastron are, except in Dermochelys, Trionyx and their allies, covered with an epidermal exoskeleton of horny plates, which are regularly arranged, though their outlines do not coincide with those of the underlying bones. The thoracic vertebrae have no transverse processes, and are quite immovably fixed, but the cervical and caudal vertebrae are very freely movable. There are no lumbar vertebrae. The skull is extremely solid, and frequently has a very complete false roof. Teeth have been detected in embryos of Trionyx but with this exception the jaws are toothless, and are encased in horny beaks. The quadrate is firmly fixed. The facial part of the skull is very short, and the alisphenoidal and orbitosphenoidal regions are unossified. In living forms there are no separate nasal bones, while large prefrontals and postfrontals are developed. There is a comparatively complete bony palate chiefly formed of the palatines and pterygoids. The anterior nares are united and placed at the anterior end of the skull, and the premaxillae are very small. There is no transpalatine bone and the vomer is unpaired. The dentaries are generally fused together.

There are ten pairs of ribs, and each rib has only a single head and is partially attached to two vertebrae; there are no cervical or sternal ribs. There is no true sternum.

The three anterior elements of the plastron are respectively homologous with the interclavicle and two clavicles of other reptiles, while the remaining elements of the plastron are probably homologous with the abdominal ribs of Crocodiles. The pectoral girdle lies within the ribs, and the precoracoids and coracoids do not meet in ventral symphyses. The scapula and precoracoid are ossified continuously. The pubis probably corresponds with the pre-pubis of Dinosaurs. There are four limbs each with five digits.

The order includes three suborders:—

Suborder (1). Trionychia.

The carapace and plastron have a rough granular surface covered with skin and without any horny shields.

The plastron is imperfectly ossified, and marginal bones may be absent, or if present are confined to the posterior portion of the carapace. The pelvis is not united to the plastron. The cranium has not a complete false roof and the head can be drawn back under the carapace.

The first three digits of both manus and pes bear claws, and the fourth digit in each case has more than three phalanges. The most important genus is Trionyx.

Suborder (2). Cryptodira.

The carapace and plastron vary in the extent to which they are ossified, and except in Dermochelys[72] and its allies are covered by horny plates. Marginal bones are always present. The head can generally be drawn back under the carapace. The pelvis is not firmly united to the plastron. The cranium often has a complete false roof, and in the mandibular articulation the cup is borne by the cranium, and the knob by the mandible. Among the more important genera are Dermochelys, Chelone, and Testudo.

Suborder (3). Pleurodira.

The carapace and plastron are strongly ossified, and firmly united to the pelvis. The head and neck can be folded laterally under the carapace, but cannot be drawn back under it. The cranium has a more or less complete false roof, and in the mandibular articulation the knob is borne by the cranium, and the cup by the mandible. Chelys is a well-known genus.

Order 4. Ichthyosauria[73].

The order includes a number of large extinct marine reptiles whose general shape is similar to that of the Cetacea. The skull is enormously large, and the neck short. The tail is very long, and is terminated by a large vertically-placed bilobed fin, the vertebral column running along the lower lobe. The very numerous vertebrae are short and deeply biconcave. The vertebral column can be divided into caudal and precaudal regions only, as the ribs which begin at the anterior part of the neck are continued to the posterior end of the trunk without being connected with any sternum or sacrum. The precaudal vertebrae bear two surfaces for the articulation of the ribs, while in the caudal vertebrae the two surfaces have coalesced. The caudal region is also distinguished by its chevron bones. The vertebrae have no transverse processes, and the neural arches are not firmly united to the centra, and have only traces of zygapophyses. The atlas and axis are similar to the other vertebrae, but there is a wedge-shaped intercentrum between the atlas and the skull, and another between the atlas and the axis. The skull is greatly elongated (fig. 32) and pointed, mainly owing to the length of the premaxillae. The orbits are enormous, and there is a ring of bones in the sclerotic (fig. 32, 15). The anterior nares are very small; and are placed far back just in front of the orbits. There is an interparietal foramen, and the supratemporal fossae (fig. 32, 9) are very large, while there are no infratemporal fossae. An epipterygoid occurs. The quadrate is firmly fixed to the cranium, and there is a large parasphenoid. There are large prefrontals, but the frontals are very small. The very numerous teeth are large and conical, and are placed in continuous grooves without being ankylosed to the bone. They are confined to the jaw-bones.

Fig. 32. Lateral (below) and dorsal (above) views of the skull of an Ichthyosaurus. (Modified from Deslongchamps.)

1. premaxillae.	12. squamosal.
2. maxillae.	13. supratemporal.
3. nasal.	14. quadratojugal.
4. prefrontal[74].	15. sclerotic ring.
5. frontal.	16. postorbital.
6. postfrontal[74].	17. jugal.
7. anterior nares.	18. lachrymal.
8. orbit.	19. dentary.
9. supratemporal fossa.	20. articular.
10. interparietal foramen.	21. angular.
11. parietal.

The ribs are long, and the anterior ones have capitula and tubercula. There is no sternum, but the ventral body wall is strengthened by a complex system of abdominal splint ribs.

The pectoral girdle is strongly developed, the scapulae are narrow, the coracoids broad, and meet ventrally without overlapping. There are probably no precoracoids, but clavicles and a T-shaped interclavicle are well developed.

The limbs are very short, and completely modified into swimming paddles. The humerus and femur are both short, while the radius and ulna, tibia and fibula are generally still further reduced to the form of short polygonal bones.

The digits are formed of longitudinal series of very numerous small bones. The number of digits is five, but there sometimes appear to be more owing to the bifurcation of certain of them, or to the addition of marginal bones, either to the radial or ulnar side of the limb. The humerus has no foramen, and both humerus and femur are unique in that they are distally terminated by concave surfaces instead of by convex condyles. The pelvic limb is much smaller than the pectoral. The pelvis has no bony connection with the vertebral column, and all the component bones are small and rod-like.

The Ichthyosauria are confined to beds of Secondary age and by far the best known genus is Ichthyosaurus.

Order 5. Rhynchocephalia.

This order includes the living Sphenodon (Hatteria) and various extinct forms. The general shape of these animals is lizard-like and the tail is long.

The vertebrae are amphicoelous or sometimes nearly flat, and the notochord sometimes persists to some extent. Proterosaurus differs from the other members of the order in having opisthocoelous cervical vertebrae.

The sacrum is composed of two vertebrae. Ossified inter centra (interdorsalia) generally occur in the cervical and caudal regions, and sometimes throughout the whole vertebral column. In the skull the quadrate is immovably fixed and united to the pterygoid. The palate is well ossified, while the premaxillae which are often beak-like are never ankylosed together. The jaws may be toothless or may be provided with teeth which are usually acrodont (see p. 199). The palatines frequently bear teeth, and in Proterosaurus teeth occur also on the pterygoids and vomers. The rami of the mandible are united by ligament at the symphysis except in the Rhynchosauridae, in which the union is bony. Superior and inferior temporal arcades occur.

The ribs have capitula and tubercula, and often uncinate processes (see p. 190) as in birds. A pectoral girdle and sternum, with clavicles and a T-shaped interclavicle are developed, and abdominal ribs are always found. The precoracoid is however absent. The limbs are pentedactylate.

Sphenodon[75] (Hatteria) now living in some of the islands of the New Zealand group, is certainly the most generalised of all living reptiles. Though lizard-like in form it differs from all living lizards in the possession of two temporal arcades, abdominal ribs and a fixed quadrate; and is often considered to be nearly allied in many respects to the type of reptile from which all the others took their origin.

Among the better known extinct forms are Proterosaurus of Permian and Hyperodapedon of Triassic age.

Order 6. Squamata.

This order includes the extinct Mosasaurians, and the lizards and snakes which form the vast majority of living reptiles. The trunk may be moderately elongated and provided with four short limbs as in lizards, or it may be limbless, extremely elongated, and passing imperceptibly into the tail. The surface is generally completely covered with overlapping horny epidermal scales, below which bony dermal scutes may be developed.

The vertebrae are procoelous, rarely amphicoelous. There are no inter centra, and the neural arches are firmly united to the centra. Additional articulating surfaces, the zygosphenes and zygantra, are often developed[76]. The sacrum is formed of two or rarely three vertebrae, or may be wanting as in Ophidia. In the skull an infratemporal arcade forming the lower boundary of the infratemporal fossa is absent, and the quadrate, except in the Chamaeleons, is movably articulated to the squamosal. The palatal vacuities are large and the nares are separate. There is often a distinct parasphenoid. The teeth are either acrodont (i.e. ankylosed to the summit of the jaw), or pleurodont, i.e. ankylosed to the inner side of the jaw. The thoracic ribs each have a single head which articulates with the centrum of the vertebra; while uncinate processes and abdominal ribs never occur.

A pectoral girdle and sternum may be present, or may be completely absent as in snakes. Except in snakes there are generally four pentedactylate limbs which may either form paddles or be adapted for walking.

Suborder (1). Lacertilia[77].

The body is elongated, and as a rule four short pentedactylate limbs are present, but sometimes limbs are vestigial or absent. The exoskeleton generally has the form of horny plates, spines, or scales; while sometimes as in the Chamaeleons and Amphisbaenians it is absent. In other forms such as Tiliqua and Scincus, the body has a complete armour of bony scutes, whose shape corresponds with that of the overlying horny scales.

The vertebrae are procoelous, rarely as in the Geckos amphicoelous; they are usually without zygosphenes and zygantra, but these structures occur in the Iguanidae. The sacral vertebrae of living forms are not ankylosed together, and the caudal vertebrae usually have well-developed chevron bones.

In the skull[78] the orbits are separated from one another, only by an imperfectly developed interorbital septum, the cranial cavity not extending forwards between them, while the alisphenoidal region is unossified. The premaxillae may be paired or united (Amphisbaenidae), and there is usually an interparietal foramen. There may be a complete supratemporal[79] arcade bounding the lower margin of the supratemporal fossa, or the supratemporal fossa may be open below. The quadratojugal is not ossified, and the quadrate articulates with the exoccipital. There is no infratemporal arcade. There is commonly a rod-like epipterygoid[80] (fig. 33, 14) connecting the pterygoid and parietal.

Teeth are always present, and may be confined to the jaws or may be developed also on the pterygoids and rarely on the palatines; they are either acrodont or pleurodont. The rami of the mandible are suturally united.

A pectoral girdle is always present, and generally also a sternum. Clavicles and a T-shaped interclavicle are commonly present, but are absent in the Chamaeleons.

Fig. 33. A, Lateral view, and B, longitudinal section of the skull of a Lizard (Varanus varius). × 3/5. (Brit. Mus.)

1. premaxillae.	16. transpalatine.
2. maxillae.	17. parasphenoid.
3. nasal.	18. quadrate.
4. lateral ethmoid.	19. parietal.
5. supra-orbital.	20. squamosal.
6. lachrymal.	21. supratemporal.
7. frontal.	22. exoccipital.
8. postfrontal.	23. dentary.
9. prefrontal.	24. splenial.
10. basisphenoid.	25. supra-angular.
11. pro-otic.	26. angular.
12. epi-otic.	27. coronoid.
13. pterygoid.	28. articular.
14. epipterygoid (columella	29. vomer.
cranii).	30. basi-occipital.
15. jugal.	31. orbitosphenoid.

There is no separate precoracoid but a precoracoidal process (fig. 34, 7) of the coracoid is generally prominent.

Fig. 34. Lateral view of the shoulder-girdle of Varanus. × 3/5. (Brit. Mus.).

1. suprascapula.	5. clavicle.
2. scapula.	6. interclavicle.
3. glenoid cavity.	7. precoracoidal process.
4. coracoid.

Sternal ribs are present in chamaeleons and scinks. The limbs are in the great majority of cases pentedactylate and the digits are clawed. The phalanges articulate by means of condyles. Sometimes one or both pairs of limbs are absent. When the posterior limbs are absent the pelvis is also wanting, though the loss of the anterior limbs does not lead to a corresponding loss of the pectoral girdle.

The pubis corresponds to the pre-pubis of Dinosaurs, and both pubes and ischia meet in ventral symphyses.

The suborder includes the Lizards, Chamaeleons and Amphisbaenians.

Suborder (2). Ophidia[81].

The Ophidia or snakes are characterised by their greatly elongated body and want of limbs. The body is covered with overlapping horny scales and bony dermal scutes are never present. The vertebrae are procoelous, and are distinguishable into two groups only, precaudal or rib-bearing, and caudal or ribless. The atlas vertebra is also ribless. The neural arches are always provided with zygosphenes and zygantra. Many of the vertebrae have strong hypapophyses, and the caudal vertebrae are without chevron bones.

In the skull the cranial cavity extends forwards between the orbits, and is closed in front by downgrowths from the frontals and parietals which meet the well-ossified alisphenoids and orbitosphenoids[82]. The cranium is strongly ossified, and there are no parotic processes or interparietal foramen. There are no temporal arcades and no epipterygoid. The premaxillae if present are very small (fig. 51, 1) and usually toothless. The quadrates articulate with the squamosals, and do not as in Lacertilia meet the exoccipitals. The palatines do not unite directly with the vomers or with the base of the cranium, and the whole palato-maxillary apparatus is more loosely connected with the cranium than it is in Lacertilia. The pterygoids, and in most cases also the palatines, bear teeth. The dentition is acrodont, and the rami of the mandible are united only by an elastic ligament—an important point serving to distinguish the Ophidia from the Lacertilia. There is an imperfectly developed interorbital septum, the ventral part of which is formed by the parasphenoid. The postfrontal is generally well developed, while the jugals and quadratojugals are absent. There are never any traces of the anterior limbs or pectoral girdle, but occasionally there are vestiges of a pelvis and posterior limbs.

Suborder (3). Pythonomorpha[83].

This suborder includes Mosasaurus and its allies, a group of enormous extinct marine reptiles found in beds of Cretaceous age.

The skin is in most forms at any rate unprovided with dermal scutes. The vertebrae may be with or without zygosphenes and zygantra. The skull resembles that of lizards, having an interparietal foramen, and a cranial cavity open in front. The squamosal takes part in the formation of the cranial wall, and the quadrate articulates with the squamosal, not as in Lacertilia with the exoccipital. There are large supratemporal fossae, bounded below by supratemporal arcades. The teeth are large and acrodont, and occur on the pterygoids as well as on the jaws. The two rami of the mandible are united by ligament only. Pectoral and pelvic girdles are present, but clavicles are wanting, and the pelvis is not as a rule united to any sacrum.

The limbs are pentedactylate, and are adapted for swimming, while all the limb bones except the phalanges are relatively very short. The number of phalanges is not increased beyond the normal, and they articulate with one another by flat surfaces. The terminal phalanges are without claws.

Order 7. Dinosauria[84].

The extinct reptiles comprising this order were all terrestrial, and include the largest terrestrial animals known. They vary greatly in size and in the structure of the limbs, some approach close to the type of structure met with in birds, others are allied to crocodiles.

Passing to the more detailed characters:—there is sometimes a well-developed exoskeleton having the form of bony plates or spines. The vertebrae may be solid or their centra may be hollowed internally; their surfaces may be flat, biconcave or opisthocoelous. The sacrum is composed of from two to six vertebrae.

As regards the skull, the quadrate is large and fixed, and supratemporal and infratemporal fossae bounded by bone occur. The teeth are more or less laterally compressed, and often have serrated edges; they may be placed in distinct sockets or in a continuous groove. The ribs have capitula and tubercula, and sternal ribs often occur. The scapula is very large, the coracoid small, and there is no precoracoid, or T-shaped interclavicle. Clavicles are only known in a few cases. In the pelvis the ilium is elongated both in front of, and behind, the acetabulum, sometimes the pre-pubis, sometimes the post-pubis is the better developed. The anterior limbs are shorter than the posterior, and the long bones are sometimes solid, sometimes hollow.

There are three well-marked suborders of the Dinosauria.

Suborder (1). Sauropoda[85].

The reptiles belonging to this group were probably quadrupedal and herbivorous.

They have the cervical and anterior trunk vertebrae opisthocoelous, while the posterior vertebrae are biconcave; all the presacral, and sometimes the sacral vertebrae are hollowed internally. The teeth are spatulate and without serrated edges, they are always planted in distinct sockets, and some of them are borne by the premaxillae.

Fig. 35. Restored skeleton of Ceratosaurus nasicornis. × 1/30. (After Marsh.)

1. anterior nares.	5. scapula.
2. prominence on the nasal	6. coracoid.
bones which probably	7. ilium.
carried a horn.	8. pubis (pre-pubis).
3. pre-orbital vacuity.	9. ischium.
4. orbit.

The nares have the form of long slits and there are large pre-orbital vacuities.

The limb bones are solid, and the anterior limbs are not much shorter than the posterior ones. All the limbs are plantigrade and pentedactylate, and the digits of the pes are clawed. There is a large pre-pubis directed downwards and forwards, meeting its fellow in a ventral symphysis, but there is no post-pubis.

The Sauropoda are found in the secondary rocks of Europe and N. America and include the largest land animals that are known to have existed. Many of the best known forms such as Brontosaurus and Morosaurus are North American.

Suborder (2). Theropoda.

The members of this suborder were all carnivorous, and from the small comparative size of the anterior limbs many of them were probably bipedal.

The vertebrae are opisthocoelous or amphicoelous, their neural arches are provided with zygosphenes and zygantra, and their centra are frequently hollowed internally; the limb bones are also hollow, and in fact the whole skeleton is extremely light. The tail is of great length. The teeth are pointed and recurved, and have one or both borders serrated; they are always planted in distinct sockets, and some of them are borne by the premaxillae. There are large pre-orbital vacuities. The digits of both manus and pes are terminated by pointed ungual phalanges which must have borne claws. In the pelvis the pre-pubes and ischia are slender bones, the former meeting in a ventral symphysis. The ilia are very deep vertically and there are no post-pubes. The astragalus is closely applied to the tibia, in front of which it sends an ascending process, sometimes the two bones appear to have been ankylosed together, as in birds. The metatarsals are elongated and the feet digitigrade.

The Theropoda vary greatly in size, one of the best known genera Compsognathus was about as large as a cat, another, Megalosaurus, perhaps as large as an elephant. Ceratosaurus is the name of a well-known North American form regarded by many authorities as identical with Megalosaurus.

Suborder (3). Orthopoda.

This suborder includes the most specialised of the Dinosaurs, certain of which resemble the Theropoda in being bipedal. In some of them such as Stegosaurus the exoskeleton is strongly developed, in others such as Iguanodon it is absent.

The vertebrae are solid and may be opisthocoelous, biconcave, or flat. The teeth are compressed and serrated, often irregularly, and are frequently not set in distinct sockets. The anterior part of the premaxillae is without teeth, and a toothless predentary or mento-meckelian bone is present. The pre-orbital vacuities are small or absent, and the nares are large and placed far forwards.

The most characteristic features of the group are found in the pelvis which, except in the Ceratopsia, bears a striking resemblance to that of birds. The ischium and post-pubis are long slender bones directed backwards parallel to one another, and the pre-pubis is also well developed. The ischium has an obturator process. The limb bones are sometimes hollow, sometimes solid. The anterior limbs are much shorter than the posterior, pointing to a bipedal method of progression. The pes is digitigrade or plantigrade, and has three, rarely four, digits.

The suborder Orthopoda may be further subdivided into three sections:—

A. Stegosauria.

A dermal exoskeleton is strongly developed. The vertebral centra are flat or biconcave, and neither they nor the limb bones are hollowed out by internal cavities. The limbs are plantigrade, the anterior ones short, the posterior ones very large and strong. The post-pubis is well developed;

e.g. Stegosaurus from the Upper Jurassic of Colorado.

B. Ceratopsia.

There is sometimes a well-developed dermal exoskeleton formed of small granules and plates of bone. The bones are solid, and the vertebral centra flat. The cranium bears a pair of enormous pointed frontal horns, and the parietal is greatly expanded and elevated behind, forming with the squamosals a shield which overhangs the anterior cervical vertebrae. The premaxillae are united, and in front of them is a pointed beak-like bone which bites upon a toothless predentary ossification of the mandible. The teeth have two roots. The anterior limbs are but little shorter than the posterior ones. There is no post-pubis;

e.g. Polyonax from the uppermost Cretaceous of Montana.

C. Ornithopoda[86].

There is no dermal exoskeleton. The cervical vertebrae are opisthocoelous, and so are sometimes the thoracic. The limb bones are hollow and the anterior limbs are much shorter than the posterior ones. The feet are digitigrade and provided with long pointed claws. The post-pubis is long and slender and directed back parallel to the ischium;

e.g. Iguanodon from the European Cretaceous.

Order 8. Crocodilia[87].

This order includes the Crocodiles, Alligators and Garials and various extinct forms, some of which are closely allied to the early Dinosaurs.

There is always a more or less complete exoskeleton formed of bony scutes overlain by epidermal scales; these bony scutes are specially well developed on the dorsal surface but may occur also on the ventral. The vertebral column is divisible into the five regions commonly distinguishable. In all living forms the vertebrae, with the exception of the atlas and axis, the two sacrals, and first caudal, are procoelous, but in many extinct forms they are amphicoelous. The atlas (fig. 71) is remarkable, consisting of four pieces, and the first caudal is biconvex.

The teeth are, in the adult, planted in separate deep sockets. The skull is very dense and solid, and all the component bones including the quadrate are firmly united. The dorsal surface of the skull is generally characteristically sculptured. There is an interorbital septum, and the orbitosphenoidal and presphenoidal regions are imperfectly ossified. Supratemporal, infratemporal, and post-temporal fossae occur, but no interparietal foramen. In living genera there is a long secondary palate formed by the meeting in the middle line of the palatines, pterygoids and maxillae (fig. 43, A).

Cervical ribs (fig. 41, 8 and 9) are well developed, and articulate with rather prominent surfaces borne on the neural arches and centra respectively. The thoracic ribs articulate with the long transverse processes, and sternal ribs and abdominal splint ribs (fig. 46, 4) occur. The sternum is cartilaginous, and both it and the shoulder-girdle are very simple. The precoracoid is represented by merely a small process on the coracoid, while the clavicles are absent, except in the Parasuchia. In the pelvis (fig. 49) there is a large ilium, and an ischium meeting its fellow in a ventral symphysis; these two bones form almost the whole of the acetabulum. In front of the acetabulum, in the Eusuchia, projects a bone which is generally called the pubis, but is in reality rather an epipubis (fig. 49, 4), the true pubis being probably represented by a fourth element which remains cartilaginous for some time, and later on ossifies and attaches itself to the ischium. The limbs are small in proportion to the size of the body, and are adapted for swimming or for shuffling along the ground; they are plantigrade and the bones are all solid. In living forms the anterior limbs have five digits and the posterior four, the fifth being represented only by a short metatarsal. The first three digits in each case are clawed. The calcaneum has a large backwardly-projecting process.

The order Crocodilia may be subdivided into two suborders.

Suborder (1). Parasuchia.

The vertebral centra are flat or biconcave. The premaxillae are very large, and the nares are separated, and placed far back. The posterior narial openings lie comparatively far forward between the anterior extremities of the palatines.

The palatines and pterygoids do not form a secondary palate. The supratemporal fossae are small, and open posteriorly, the lateral temporal fossae are very large. The parietals and frontals are paired. Clavicles are present. The best known and most important genus of these extinct crocodiles is Belodon.

Suborder (2). Eusuchia.

The vertebrae are either biconcave or procoelous. The premaxillae are small, and the anterior nares are united and placed far forwards. The posterior nares lie far back, the palatines and in living genera the pterygoids, meeting in the middle line, and giving rise to a closed palate. The supratemporal fossae are surrounded by bone on all sides, and the parietals, and often also the frontals are united. There are no clavicles. The suborder includes the genera Crocodilus, Alligator, Garialis and others living and extinct.

Order 9. Pterosauria[88].

These animals, called also the pterodactyles or Ornithosauria, are a group of extinct reptiles, whose structure has been greatly modified from the ordinary reptilian type for the purpose of flight.

The skin was naked and they vary greatly in size and in the length of the tail. The vertebrae and limb bones are pneumatic just as in birds. The presacral vertebrae are procoelous and have their neural arches firmly united to the centra. The neck is long, the caudal vertebrae are amphicoelous, and from three to five vertebrae are fused together in the sacral region. The skull is large and somewhat bird-like, the facial portion being much drawn out anteriorly, and the sutures being obliterated. It resembles that of other reptiles in having large supratemporal fossae; large pre-orbital vacuities also occur. The jaws may be toothed or toothless, and the teeth, when present, are imbedded in separate sockets. The premaxillae are large, and the quadrate is firmly attached to the skull. The rami of the mandible are united at the symphysis, and there is an ossified ring in the sclerotic. The occurrence of a postfrontal and its union with the jugal behind the orbit, are characteristic reptilian features.

The ribs have capitula and tubercula, and sternal and abdominal ribs occur. The sternum has a well-developed keel, and the scapula and coracoid are large and bird-like. There are no clavicles or interclavicle.

The anterior limbs are modified to form wings by the great elongation of the fifth digit, to which a membrane was attached. The second, third and fourth digits are clawed and are not elongated in the way that they are in bats. The pollex, if present at all, is quite vestigial.

The pelvis is weak and small, and though the ilia are produced both in front of and behind the acetabula, in other features the pelvis is not bird-like. The ischia are short and wide, and the pubes are represented only by the pre-pubes. The posterior limbs are small and the fibula is much reduced. The pes is quite reptilian in type, and has five separate slender metatarsals. The two best known genera are Pterodactylus, in which the tail is short, and Rhamphorhynchus, in which it is long. The Pterosauria are found throughout the Jurassic and Cretaceous formations in both Europe and North America.

The most striking feature as regards the skeleton of the Turtle is that the trunk is enveloped in a bony box, the dorsal portion of which is called the carapace, while the ventral portion is the plastron.

I. EXOSKELETON.

a. The epidermal exoskeleton in the Green Turtle as in all other Chelonia except Dermochelys, Trionyx and their allies is strongly developed, its most important part consisting of a series of horny shields which cover over the bony plates of the carapace and plastron but do not at all correspond to them in size and arrangement.

The shields covering over the carapace consist of three rows of larger central shields,—five (vertebral) shields being included in the middle row and four (costal) in each lateral row,—and of a number of smaller marginal shields.

Of the marginal shields, that lying immediately in front of the first vertebral is termed the nuchal, while the two succeeding the last vertebral are called sometimes pygal, sometimes supracaudal; the remainder are the marginal shields proper.

The epidermal covering of the plastron consists principally of six pairs of symmetrically arranged shields, called respectively the gular, humeral, pectoral, abdominal, femoral, and anal, the gular being the most anterior. In front of the gular shields is an unpaired intergular, and the shields of the plastron are connected laterally with those of the carapace, by five or six pairs of rather irregular infra-marginal shields. Smaller horny plates occur on other parts of the body, especially on the limbs and head.

Two other sets of structures belong also to the epidermal exoskeleton, viz. (a) horny beaks with denticulated edges which ensheath both upper and lower jaws, (b) claws, which as a rule are borne only by the first digit of each limb. Sometimes in young individuals the second digit is also clawed.

b. The dermal exoskeleton is strongly developed, and is combined with endoskeletal structures derived from the ribs and vertebrae to form the carapace.

The Carapace (fig. 36) consists of a number of plates firmly united to one another by sutures. They have a very definite arrangement and include:

(a) the nuchal plate (fig. 36, 1), a wide plate forming the whole of the anterior margin of the carapace. It is succeeded by three series of plates, eight in each series, which together make up the main part of the carapace. Of these the small

(b) neural plates[1] (fig. 36, A, 2) form the middle series. They are closely united with the neural arches of the underlying vertebrae;

(c) the costal plates[89] (fig. 36, A, 3) are broad arched plates united to one another by long straight sutures. They are united at their inner extremities with the neural plates, but the boundaries of the two sets of plates do not regularly correspond. Each is united ventrally with a rib which projects beyond it laterally for some distance; (d) the marginal plates (fig. 36, 4) are twenty-three in number, eleven lying on each side, while an unpaired one lies in the middle line posteriorly. Many of them are marked by slight depressions into which the ends of the ribs fit; (e) the pygal plates (fig. 36, 5) are two unpaired plates lying immediately posterior to the last neural.

Fig. 36. A, dorsal and B, ventral view of the carapace of a Loggerhead Turtle (Thalassochelys caretta), (after Owen).

1. nuchal plate.	6. rib.
2. first neural plate.	7. thoracic vertebra.
3. second costal plate.	8. first vertebral shield.
4. marginal plate.	9. costal shield.
5. pygal plate.

The sculpturing due to the epidermal shields is very obvious on the carapace.

The plastron (fig. 37) consists of one unpaired ossification, the entoplastron, and four pairs of ossifications called respectively the epiplastra, hyoplastra, hypoplastra, and xiphiplastra.

The epiplastra (fig. 37, 1) are the most anterior, they are expanded and united to one another in the middle line in front, while behind each tapers to a point which lies external to a process projecting forwards from the hyoplastron. They are homologous with the clavicles of other vertebrates.

The entoplastron or episternum (fig. 37, 2) which is homologous with the interclavicle of other reptiles, is expanded at its anterior end and attached to the symphysis of the epiplastra, while behind it tapers to a point and ends freely.

The hyoplastra are large irregular bones each closely united posteriorly with the corresponding hypoplastron, and drawn out anteriorly into a process which lies internal to that projecting backwards from the epiplastron. Each gives off on its inner surface a slender process which nearly meets its fellow, while the anterior half of the outer surface is drawn out into several diverging processes.

The hypoplastra (fig. 37, 4) are flattened bones resembling the hyoplastra, with which they are united by long sutures; the posterior half of both outer and inner surfaces is drawn out into a number of pointed processes.

The xiphiplastra are small flattened elongated bones meeting one another in the middle line posteriorly. In front they are notched and each interlocks with a process from the hypoplastron of its side. The hyoplastra, hypoplastra and xiphiplastra are homologous with the abdominal ribs of Crocodiles.

Fig. 37. The Plastron of a Green Turtle (Chelone midas). × 1/7. (Camb. Mus.)

1. epiplastron (clavicle).	4. hypoplastron.
2. entoplastron (interclavicle).	5. xiphiplastron.
3. hyoplastron.

II. ENDOSKELETON.

1. The Axial Skeleton.

The axial skeleton includes the vertebral column, the ribs, and the skull.

A. The Vertebral column and Ribs.

The number of vertebrae in the Green Turtle is thirty-eight, not a great number as compared with that in many reptiles, and of these eighteen are caudal.

The vertebral column is divisible into four regions only—cervical, thoracic, sacral, and caudal.

The Cervical vertebrae.

These are eight in number, and are chiefly remarkable for the great variety of articulating surfaces which their centra present, and for their mobility upon one another.

The first or atlas vertebra differs much from all the others and consists of the following parts:—

a. the neural arch, formed of two separate ossifications united in the mid-dorsal line;

b. the inferior arch;

c. the centrum, which is detached from the rest and forms the odontoid process of the second vertebra.

Each half of the neural arch consists of a ventral portion, the pedicel, which lies more or less vertically and is united ventrally to the inferior arch, and of a dorsal portion, the lamina, which lies more or less horizontally and meets its fellow in the middle line in front, partially roofing over the neural canal. Each pedicel bears a facet on its anterior surface, which, with a corresponding one on the inferior arch, articulates with the occipital condyle of the skull. Three similar facets occur also on the posterior surface of the pedicel and inferior arch, and articulate with the odontoid process. The laminae meet one another in front, but do not fuse, while behind they are separated by a wide triangular space. They bear a pair of small downwardly-directed facets, the postzygapophyses, for articulation with the prezygapophyses of the second vertebra.

The inferior arch is a short irregular bone bearing two converging facets for articulation with the occipital condyle and odontoid process respectively.

The centrum or odontoid process has a convex anterior surface for articulation with the neural and inferior arches, and a concave posterior surface by which it is united with the centrum of the second or axis vertebra. It bears posteriorly a small epiphysis which is really a detached portion of the inferior arch.

The second or axis and following five cervical vertebrae, though showing distinct differences, resemble one another considerably, each having a fairly elongated centrum with a keel-like hypapophysis, each having also a neural arch with prominent articulating surfaces, the anterior of which, or prezygapophyses, look upwards and inwards, while the posterior ones, the postzygapophyses, look downwards and outwards. They however, as was previously mentioned, differ very remarkably in the character of the articulating surfaces of the centra. Thus the second and third vertebrae are convex in front and concave behind, the fourth is biconvex, the fifth is concave in front and convex behind. The sixth is concave in front and attached to the seventh by a flat surface behind, the seventh has a flat anterior face and two slightly convex facets behind. The vertebrae all have short blunt transverse processes and the second has a prominent neural spine.

The eighth cervical vertebra is curiously modified, the centrum is very short, has a rather prominent hypapophysis, and is convex behind, while in front it articulates with the preceding centrum by two concave surfaces. The neural arch is deeply notched in front and bears two upwardly-directed prezygapophyses, while behind it is very massive and is drawn out far beyond the centrum, bearing a pair of flat postzygapophyses. The top of the neural arch almost or quite meets a blunt outgrowth from the nuchal plate.

The Thoracic vertebrae.

These are ten in number and are all firmly united with the ribs and elements forming the carapace.

The first thoracic vertebra differs from the others, the centrum is short and has a concave anterior surface articulating with the centrum of the last cervical vertebra, and a pair of prezygapophyses borne on long outgrowths. The neural spine arises only from the anterior half of the centrum, and is not fused to the carapace. Arising laterally from the anterior part of the centrum are a small pair of ribs each of which is connected with a process arising from the rib of the succeeding vertebra.

The next seven thoracic vertebrae are all very similar, each has a long cylindrical centrum, expanded at the ends, and firmly united to the preceding and succeeding vertebrae. The neural arches are flattened and expanded dorsally, and are united to one another and to the overlying neural plates; each arises only from the anterior half of its respective centrum, and overlaps the centrum of the vertebra in front of it. Between the base of the neural arch and its successor is a small foramen for the exit of the spinal nerve. There are no transverse processes or zygapophyses.

To each thoracic vertebra from the second to ninth inclusive, there corresponds a pair of ribs (fig. 36, 6) of a rather special character. Each is suturally united with the anterior half of the edge of its own vertebra, and overlaps on to the posterior half of the edge of the next preceding vertebra. The ribs are much flattened, and each is fused with the corresponding costal plate, beyond which it projects to fit into a pit in one of the marginal plates.

The tenth thoracic vertebra is smaller than the others, and its neural arch does not overlap the preceding vertebra, it bears a pair of small ribs which are without costal plates, but meet those of the ninth vertebra.

There are no lumbar vertebrae.

The Sacral vertebrae.

The sacral vertebrae are two in number, they are short and wide, their centra are ankylosed together, and their neural arches are not united to the carapace.

The first has the anterior face of the centrum concave and the posterior flat, while both faces of the second are flat. Each bears a pair of short ribs which meet the ilia, but are not completely ankylosed either with them or the centra.

The Caudal vertebrae.

The caudal vertebrae are eighteen in number. The centrum of the first is flat in front and is ankylosed to the second sacral; behind it is convex. The others are all very similar to one another, and decrease gradually in size when followed back. Each has a moderately long centrum, concave in front and convex behind, both terminations being formed by epiphyses. The neural arch arises only from the anterior half of the vertebra; it bears a blunt truncated neural spine and prominent pre- and post-zygapophyses. The first seven caudal vertebrae bear short ribs attached to their lateral margins, the similar outgrowths on the succeeding vertebrae do not ossify from distinct centres, and are transverse processes rather than ribs.

B. The Skull.

The skull of the Turtle is divisible into the following three parts:—

(1) the cranium;

(2) the lower jaw or mandible;

(3) the hyoid.

(1) The Cranium.

The cranium is a very compact bony box, containing a cavity in which the brain lies, and which is a direct continuation of the neural canal of the vertebrae.

Fig. 38. The skull of the Green Turtle (Chelone midas). × ½. A, posterior half, B, anterior half. (Brit. Mus.)

1. parietal.	13. angular.
2. squamosal.	14. supra-angular.
3. quadrate.	15. premaxillae.
4. basisphenoid.	16. maxillae.
5. basi-occipital.	17. jugal.
6. quadratojugal.	18. postfrontal.
7. opisthotic.	19. vomer.
8. exoccipital.	20. prefrontal.
9. foramen magnum.	21. frontal.
10. splenial.	22. external auditory meatus
11. articular.	leading into tympanic
12. dentary.	cavity.

Like those of the skull as a whole its component bones may be subdivided into three sets:—

1. those forming the brain-case or cranium proper;

2. those developed in connection with the special sense organs;

3. those forming the upper jaw and suspensorial apparatus.

Both cartilage and membrane bones take part in the formation of the skull, and a considerable amount of cartilage remains unossified, especially in the ethmoidal and sphenoidal regions.

1. The Cranium proper or Brain-case.

The cartilage and membrane bones of the brain-case when taken together can be seen to be more or less arranged in three rings or segments, called respectively the occipital, parietal, and frontal segments.

The occipital segment is the most posterior of these, and consists of four cartilage bones, the basi-occipital, the two exoccipitals and the supra-occipital; these bound the foramen magnum.

The basi-occipital (figs. 38 and 39, 5) lies ventral to the foramen magnum and only bounds a very small part of it; it forms one-third of the occipital condyle by which the skull articulates with the atlas vertebra. It unites dorsally with the exoccipitals and anteriorly with the basisphenoid.

The exoccipitals are rather small bones, which form the sides and the greater part of the floor of the foramen magnum, and two-thirds of the occipital condyle. Laterally each is united with the pterygoid and opisthotic of its side. At the sides of the occipital condyle each exoccipital is pierced by a pair of foramina, the more dorsal and posterior of which transmits the hypoglossal nerve.

The supra-occipital (fig. 39, 14) is a larger bone than the others of the occipital segment. It forms the upper border of the foramen magnum and is drawn out dorsally into a large crest which extends back far beyond the occipital condyle. In the adult the supra-occipital is completely ankylosed with the epi-otics.

The Parietal segment.

The ventral portion of the parietal segment is formed by the basisphenoid (figs. 38 and 39, 4) which lies immediately in front of the basi-occipital. A triangular portion of it is seen in a ventral view of the skull, but it is quickly overlapped by the pterygoids. It gives off dorsally a pair of short processes which meet the pro-otics.

The alisphenoidal region is unossified and the only other constituents of the parietal segment are the parietals (fig. 39, 1). These are large bones which, after roofing over the cranial cavity, extend upwards and become expanded into a pair of broad plates which unite with the squamosal and bones of the frontal segment to form a wide, solid, false roof to the skull. Each also sends ventralwards a plate which meets an upgrowth from the pterygoid and acts as an alisphenoid.

The Frontal segment.

Of the frontal segment the basal or presphenoidal and lateral or orbitosphenoidal portions do not become ossified, the dorsal portion however includes three pairs of membrane bones, the frontals, prefrontals and postfrontals.

The frontals are a pair of small bones lying immediately in front of the parietals, and in front of them are the prefrontals (figs. 38 and 39, 20), a pair of similar but still smaller bones, which are produced ventrally to meet the vomer and palatines. They form also the dorsal boundary of the anterior nares. The postfrontals (figs. 38 and 39, 18) are larger bones, united dorsally to the frontals and parietals, posteriorly to the squamosals, and ventrally to the jugals and quadratojugals. All three pairs of frontal bones, especially the postfrontals, take part in the bounding of the orbits.

Fig. 39. Longitudinal vertical section through the Cranium of a Green Turtle (Chelone midas). × 2/3. (Camb. Mus.)

1. parietal.	10. palatine.	17. jugal.
2. squamosal.	11. rod passed into narial	18. postfrontal.
3. quadrate.	passage.	19. vomer.
4. basisphenoid.	12. exoccipital.	20. prefrontal.
5. basi-occipital.	13. epi-otic fused to	21. frontal.
6. quadratojugal.	supra-occipital.	V, VII, VIII, IX, X, XI, XII,
7. pro-otic.	14. supra-occipital.	foramina for the exit of
8. opisthotic.	15. premaxillae.	cranial nerves.
9. pterygoid.	16. maxillae.

2. The Sense capsules.

Skeletal structures occur in connection with each of the three special sense organs of hearing, sight, and smell.

The Auditory capsules.

The auditory or periotic capsule of the turtle is rather large and its walls are well ossified, epi-otic, pro-otic and opisthotic bones being present.

The epi-otic (fig. 39, 13) is the more dorsal of the three bones, and in the adult is completely ankylosed with the supra-occipital.

The opisthotic (fig. 39, 8) is the ventral posterior element. On its inner side it is united to the supra-occipital above, and to the exoccipital below; it sometimes becomes completely fused with the exoccipital. In front it meets the pro-otic, and on its outer side the squamosal and quadrate. Its anterior portion is hollowed out by the cavity in which the auditory organ lies, it gives off also a process which is separated from the exoccipital by an oval foramen through which the glossopharyngeal, pneumogastric, and spinal accessory nerves leave the cranial cavity.

The pro-otic is the anterior element; it meets the supra-occipital and opisthotic posteriorly, while anteriorly it is separated from the alisphenoidal plate of the parietal and pterygoid by a large oval foramen through which the maxillary and mandibular branches of the trigeminal nerve pass out (fig. 39, V 1 & 2). It is hollowed out posteriorly by the cavity in which the auditory organ lies, and its inner wall as seen in longitudinal section is pierced by a foramen through which the external carotid artery and facial nerve leave the cranial cavity,—the nerve finally leaving the skull through a small oval foramen on the anterior face of the pro-otic near its junction with the quadrate.

Between the pro-otic and opisthotic as seen in a longitudinal section of the skull is a large opening constricted in the middle. This is the internal auditory meatus (fig. 39, VIII.). Through it the auditory nerve leaves the cranial cavity and enters the ear. The ramus vestibularis leaves through the dorsal part of the hole, the ramus cochlearis through the ventral.

The cavity of the auditory or periotic capsule communicates with the exterior by a fairly large hole, the fenestra ovalis, which lies between the opisthotic and pro-otic, and opens into a deep depression, the tympanic cavity, which is seen in a posterior view of the skull lying just external to the exoccipital. The cavity communicates with the exterior by a large opening, the external auditory meatus (fig. 38, 22).

Several other openings are seen in the tympanic cavity; through one at the extreme posterior end the pneumogastric and spinal accessory nerves leave the skull, and through another, a little further forwards, the glossopharyngeal.

The auditory ossicles consist of a long bony columella, whose inner end fits into the fenestra ovalis, while the outer end is attached to a small cartilaginous plate, the extra-columella, which is united to the tympanum.

The Optic capsules.

The skeletal structures developed in connection with the optic capsule do not become united to the skull. They consist of:—

(a) the sclerotic, a cartilaginous sheath investing the eye and bearing

(b) a ring of ten small bony scales.

There is no lachrymal bone.

The Olfactory or Nasal capsules.

The basicranial axis in front of the basisphenoid remains cartilaginous, neither presphenoid nor mesethmoid bones are developed, and the orbits in a dry skull communicate by a wide space through which the second, third, fourth, and sixth cranial nerves pass out. Separate nasal bones also do not occur, the large prefrontals extending over the area usually occupied by both nasals and lachrymals.

The only bone developed in connection with the nasal capsules is the vomer (fig. 39, 19), an unpaired bone lying ventral to the mesethmoid cartilage, and in contact laterally with the maxillae, premaxillae and palatines.

3. The Upper Jaw and suspensorial apparatus.

A number of pairs of bones are developed in connection with the upper jaw and suspensorial apparatus, one pair, the quadrates, being cartilage bones, while the rest are all membrane bones.

The squamosals (fig. 38, 2) are large bones which, lying external to the auditory bones, extend dorsalwards to meet the parietals and postfrontals, and form a large part of the false roof of the skull. They are united ventrally with the quadrates and quadratojugals.

Each quadrate (fig. 38, 3) forms the outer boundary of the tympanic cavity, and is firmly united on its inner side with the opisthotic, exoccipital, and pterygoid. Dorsally it is fixed to the squamosal and anteriorly to the quadratojugal. Its outer surface is marked by a deep recess, and it ends below in a strong condyle with which the mandible articulates. In front of the quadrates are a pair of thin plate-like bones, the quadratojugals which are united in front to the jugals or malars.

The jugals (fig. 38, 17) are also thin plate-like bones, and form part of the posterior boundary of the orbit. They are attached dorsally to the postfrontals, and anteriorly to the maxillae, while each also sends inwards a horizontal process which meets the pterygoid and palatine.

The maxillae (figs. 38 and 39, 16) are a pair of large vertically-placed bones, each drawn out ventrally into a straight, sharp, cutting edge. They form the lateral boundaries of the anterior nares, and each sends dorsalwards a process which meets the postfrontal. Each also sends inwards a horizontal palatine process, which meets the palatine and vomer, and also forms much of the floor of the narial passage.

The premaxillae (figs. 38 and 39, 15) are a pair of very small bones forming the floor of the anterior narial opening, they are wedged in between the two maxillae, and send back processes which meet the vomer and palatines.

The palatines (fig. 39, 10) are a pair of small bones firmly united with the pterygoids behind, with the maxillae and jugals externally, and with the vomer in the middle line. Each also gives off a palatine plate which unites with the expanded lower edge of the vomer, and forms the ventral boundary of the posterior nares. Anteriorly the palatines form the posterior boundary of a large foramen through which the ophthalmic branches of the fifth and seventh nerves pass to the olfactory organs.

The pterygoids (fig. 39, 9) are a pair of large bones which unite with one another by a long median suture. They are united also with the palatines in front, and with the quadrate, basisphenoid, basi-occipital, and exoccipitals behind. Each also sends dorsalwards a short alisphenoid plate which meets that from the parietal.

Piercing the posterior end of the pterygoid is the prominent opening of the carotid canal; a bristle passed into this hole emerges through a foramen lying between the pro-otic and the alisphenoid process of the pterygoid.

(2) The Lower Jaw or Mandible.

The mandible consists of one unpaired bone, formed by the fusion of the two dentaries, and five pairs of bones, called respectively the articular, angular, supra-angular, splenial and coronoid.

The fused dentaries (fig. 38, 12) form by far the largest of the bones; they constitute the flattened anterior part of the mandible, and extend back below the other bones almost to the end of the jaw.

The coronoid is the most anterior of the paired bones, it forms a prominent process to which the muscles for closing the jaw are attached.

The articular (fig. 38, 11) is expanded, and with the supra-angular forms the concave articulating surface for the quadrate.

The splenial (fig. 38, 10) is a thin plate applied to the inner surface of the posterior part of the mandible.

The angular (fig. 38, 13) is a slender plate of bone lying below the supra-angular and splenial.

(3) The Hyoid.

The hyoid apparatus is well developed, parts of the first two branchial arches being found, as well as of the hyoid proper. It consists of a more or less oblong flattened basilingual plate or body of the hyoid which represents the fused ventral ends of the hyoid and branchial arches of the embryo, and is drawn out into a point anteriorly. The greater part is formed of unossified cartilage, but at the posterior end it is bilobed, and a pair of ossified tracts occur. To its sides are attached three pairs of structures, which are portions of the hyoid and first and second branchial arches respectively.

The free part of the hyoid consists of a small piece of cartilage attached to the anterior part of the basilingual plate at its widest portion (fig. 53, 2).

The anterior cornu or free part of the first branchial arch is much the largest of the three structures. Its proximal portion adjoining the basilingual plate is cartilaginous, as is its distal end; the main part is however ossified.

The posterior cornu or free part of the second branchial arch (fig. 53, 4) consists of a short flattened cartilaginous bar arising from the bilobed posterior end of the basilingual plate.

The hyoid apparatus has no skeletal connection with the rest of the skull.

2. The Appendicular Skeleton.

This includes the skeleton of the two pairs of limbs and their girdles.

The Pectoral Girdle.

The pectoral girdle has an anomalous position, being situated internal or ventral to the ribs. It consists of three bones, a dorsal bone, the scapula, an anterior ventral bone, the precoracoid, and a posterior ventral bone, the coracoid.

The scapula is a small somewhat rod-shaped bone forming about two-thirds of the glenoid cavity. At its proximal end it is closely united with the precoracoid, the two bones ossifying continuously. It tapers away distally, and is directed dorsalwards towards the carapace.

The precoracoid forms an angle of about 130° with the scapula, with which it is completely fused at its proximal end. Its distal end is somewhat expanded and flattened, and is terminated by a fibrocartilaginous epiprecoracoid which meets its fellow. It takes no part in the formation of the glenoid cavity.

The coracoid is a large flattened blade-shaped bone forming about one-third of the glenoid cavity. It does not meet its fellow in a ventral symphysis, and is terminated by a cartilaginous epicoracoid. The glenoid articulating surfaces of both scapula and coracoid are lined by a thick pad of cartilage.

The Anterior Limb.

This is divisible into three portions, the upper arm, fore-arm and manus.

The upper arm contains a single bone, the humerus.

The humerus (fig. 40, A, 1) is a stout, nearly straight, somewhat flattened bone widely expanded at both ends. At the proximal end is the large hemispherical head, which articulates with the glenoid cavity. Behind the head the bone is drawn out into another large rounded process. Below the head the shaft bears a small outgrowth which is continuous with a larger one on the flexor surface (see p. 29). The bone is terminated distally by the trochlea, consisting of three partially distinct convex surfaces which articulate with the bones of the fore-arm.

The fore-arm includes two bones, the radius and ulna; both these are small bones, and are immovably fixed to one another proximally and distally.

The radius or pre-axial bone is the larger of the two, and is a rod-like bone terminated at either end by an epiphysis. It articulates at its proximal end with the humerus, and at its distal end with the radiale or scaphoid bone of the carpus.

The ulna (fig. 40, A, 3) or postaxial bone is shorter than the radius, and more expanded at its proximal end, where it articulates with the humerus. It articulates distally with the intermedium (lunar) and the ulnare (cuneiform) bones of the carpus. All three bones of the arm have their terminations formed by epiphyses which ossify from centres distinct from those forming the shafts.

The Manus consists of the carpus or wrist and the hand which includes the metacarpals and phalanges.

The carpus consists of a series of ten small bones, one of which, the pisiform (fig. 40, A, 10), differs from the others in being merely an ossification in the tendon of a muscle. The remaining nine bones are arranged in a proximal row of three, the ulnare (fig. 40, A, 6), intermedium, and radiale, and a distal row of five (carpalia 1-5), each of which supports one of the metacarpals. A ninth bone, the centrale (fig. 40, A, 7), is wedged in between the two rows. The ulnare, intermedium and pisiform are comparatively large flattened bones, the others are small and cubical.

The hand. This is composed of five digits, each of which consists of a metacarpal and of a varying number of phalanges.

The metacarpals. The first metacarpal (fig. 40, A, 11) is a short flattened bone, the others are all elongated and cylindrical, and are terminated proximally by slightly concave surfaces, and distally by slightly convex ones.

The phalanges. The first and fifth digits both have two phalanges, the second, third, and fourth have each three. The distal phalanx of the first digit is stout and curved, and bears a horny claw; those of the other digits are flattened and more or less pointed.

Fig. 40. A. Anterior limb of a young Hawksbill Turtle (Chelone imbricata) × ¼ (Brit. Mus.). B. Posterior limb of a large Green Turtle (Chelone midas) × 1/8 (Camb. Mus.).

1. humerus.	12. femur.
2. radius (almost hidden by the	13. tibia.
ulna).	14. fibula.
3. ulna.	15. tibiale intermedium and
4. radiale.	centrale fused.
5. intermedium.	16. fibulare.
6. ulnare.	17. tarsale 1.
7. centrale.	18. tarsale 2.
8. carpale I.	19. tarsalia 4 and 5 fused.
9. carpale IV.	20. first metatarsal.
10. pisiform.	21. fifth metatarsal.
11. first metacarpal.	I, II, III, IV, V, digits.

The Pelvic Girdle.

The pelvic girdle consists of three bones; a dorsal bone, the ilium, an anterior ventral bone, the pubis, and a posterior ventral bone, the ischium. All three bones contribute largely to the formation of the acetabulum, with which the head of the femur articulates.

The ilium is a small slightly curved bone, which unites ventrally with the pubis and ischium, and extends dorsalwards and backwards to meet the distal ends of the sacral ribs.

The pubis is the largest bone of the three; its distal end forms a wide bilobed plate, the inner lobe meeting its fellow in a median symphysis, while the other lobe or lateral process extends outwards. Attached to the symphysis in front is a cartilaginous epipubis, while behind, the two pubes are terminated by a wide rounded cartilaginous area.

The ischium, the smallest bone of the three, is flattened and like the pubis meets its fellow in a median symphysis. A narrow band of cartilage connects the symphysis pubis with the symphysis ischii, and separates the two obturator foramina from one another.

The Posterior Limb.

This is divisible into three portions, the thigh, the crus or shin, and the pes.

The thigh includes a single bone, the femur.

The femur (fig. 40, B, 12) is a short thick bone, with a prominent rounded head articulating with the acetabulum. Behind this head is a deep pit, beyond which is a roughened area corresponding with the great trochanter of mammals. The distal end is expanded and somewhat convex.

The bones of the crus or shin are the tibia and fibula. These are both straight rod-like bones with expanded terminations which closely approach one another, while elsewhere the bones diverge considerably.

The terminations of all three of the leg bones are formed by epiphyses.

The Pes consists of the tarsus or ankle, and the foot, which is made up of five digits.

The tarsus. The tarsal bones of the Turtle do not retain their primitive arrangement to such an extent as do the carpals. They are arranged in a proximal row of two and a distal row of four. Of the bones in the proximal row the postaxial one is much the smaller and is the fibulare; the larger pre-axial one (fig. 40, B, 15) represents the tibiale, intermedium, and centrale fused, and articulates with both tibia and fibula. The first three distal tarsalia are all small bones and are very similar in size, and each articulates regularly with the corresponding metatarsal. The fourth bone (fig. 40, B, 19) is much larger, and represents tarsalia 4 and 5 fused. The first two distal tarsalia articulate with the pre-axial tarsal of the proximal row, the third only with its neighbours the second, and the fused fourth and fifth. The latter articulates with both bones of the proximal row.

Each digit consists of a metatarsal and of a varying number of phalanges.

The metatarsals. The first metatarsal (fig. 40, B, 20) is broad and flattened, the second, third and fourth, are all elongated bones with nearly flat terminations formed by small epiphyses. The fifth is large and flattened, and the articular surface for the phalanx is situated somewhat laterally.

The phalanges. The first digit has two phalanges and is the stoutest of them all; its distal phalanx is sheathed in a large horny claw. The other digits, of which the third is the longest, have each three phalanges. The distal phalanges of the second and third digits are flattened and pointed and bear small horny claws.

The species chosen for description is C. palustris, a form occurring throughout the Oriental region, but the description would apply almost equally well to any of the other species of the genus Crocodilus, and with comparatively unimportant modifications to any of the living Crocodilia.

I. EXOSKELETON.

The exoskeleton of the Crocodile is strongly developed and includes elements of both epidermal and dermal origin.

a. The epidermal exoskeleton is formed of a number of horny scales or plates of variable size covering the whole surface of the body. Those covering the dorsal and ventral surfaces are oblong in shape, and are arranged in regular rows running transversely across the body. The scales covering the limbs and head are mostly smaller and less regularly arranged, and are frequently raised into a more or less obvious keel. Those covering the dorsal surface of the tail are very prominently keeled.

The epidermal exoskeleton also includes the horny claws borne by the first three digits of both manus and pes.

b. The dermal exoskeleton. This has the form of bony scutes which underlie the epidermal scales along the dorsal surface of the trunk and anterior part of the tail. Except in very young individuals the epidermal scales are rubbed off from these scutes, which consequently come to project freely on the surface of the body. Each scute is a nearly square bony plate, deeply pitted or sculptured, and marked by a strong ridge on its dorsal surface, while its ventral surface is smooth. Contiguous scutes are united to one another by interlocking sutures.

The scutes are arranged in two distinct areas, viz. (1) a small anterior nuchal shield which lies just behind the head and is formed of six large scutes more or less firmly united together, and (2) a larger posterior dorsal shield covering the whole of the back and anterior part of the tail, and formed of smaller scutes, which are arranged in regular transverse rows, and progressively diminish in size when followed back.

The teeth are exoskeletal structures, partly of dermal, partly of epidermal origin. They lie along the margins of the jaws and are confined to the premaxillae, maxillae and dentaries. They are simple conical structures, without roots; each is in the adult placed in a separate socket, and is replaced by another which as it grows comes to occupy the pulp cavity of its predecessor. In the young animal the teeth are not placed in separate sockets but in a continuous groove. This feature is met with also in the Ichthyosauria. The groove gradually becomes converted into a series of sockets by the ingrowth of transverse bars of bone. The anterior teeth are sharply pointed and slightly recurved, the posterior ones are more blunt.

The upper jaw bears about nineteen pairs of teeth, the lower jaw about fifteen pairs. The largest tooth in the upper jaw is the tenth, and in the lower jaw the fourth.

The three living families of Crocodilia, the Crocodiles, Alligators and Garials, can be readily distinguished by the characters of the first and fourth lower teeth. In Alligators both first and fourth lower teeth bite into pits in the upper jaw; in Garials they both bite into notches or grooves in the upper jaw. In Crocodiles the first tooth bites into a pit, the fourth into a notch in the upper jaw.

II. ENDOSKELETON.

1. The Axial Skeleton.

This includes the vertebral column, the skull, and the ribs and sternum.

A. The Vertebral column.

The vertebral column is very long, consisting of some sixty vertebrae. It can be divided into the usual five regions, the cervical, thoracic, lumbar, sacral, and caudal regions.

Fig. 41. First four cervical vertebrae of a Crocodile (C. vulgaris). (Partly after von Zittel.)

1. pro-atlas.	7. tubercular portion of fourth
2. lateral portion of atlas.	cervical rib.
3. odontoid process.	8. first cervical rib.
4. ventral portion of atlas.	9. second cervical rib.
5. neural spine of axis.	10. convex posterior surface of
6. postzygapophysis of	centrum of fourth vertebra.
fourth vertebra.

The Cervical vertebrae.

Counting as cervical all those vertebrae which are anterior to the first one whose ribs meet the sternum, there are nine cervical vertebrae, all of which bear ribs.

As a type of the cervical vertebrae the fifth may be taken. It has a short cylindrical centrum deeply concave in front and convex behind. From the anterior part of the ventral surface of the centrum arises a short hypapophysis, and on each side is a facet with which the lower limb (capitulum) of the cervical rib articulates. The neural arch is strongly developed and drawn out dorsally into a long neural spine, in front of which are a pair of upstanding processes bearing the prominent upwardly and inwardly directed prezygapophyses. At the sides and slightly behind the neural spine are a corresponding pair of processes bearing the postzygapophyses, which look downwards and outwards. At the point where it joins the centrum the neural arch is drawn out into a short blunt transverse process with which the upper limb (tuberculum) of the cervical rib articulates. The sides of the neural arch are slightly notched behind for the exit of the spinal nerves.

The first or atlas vertebra differs much from any of the others, and consists of four quite detached portions, a ventral arch, with two lateral portions and one dorsal. The ventral arch (fig. 41, 4) is flat below and slightly concave in front, forming together with two flattened surfaces on the lateral portions a large articulating surface for the occipital condyle of the skull. Its posterior face is bevelled off and forms with a second pair of facets on the lateral portions a surface with which the odontoid process of the second vertebra articulates. The postero-lateral surfaces of the ventral arch also bear a pair of little facets with which the cervical ribs articulate. The lateral portions are somewhat flattened and expanded, and bear in addition to those previously mentioned a pair of small downwardly directed facets, the postzygapophyses, which articulate with the prezygapophyses of the second vertebra. The dorsal portion (fig. 41, 1) is somewhat triangular in shape, and overhangs the occipital condyle. It is often regarded as the neural arch of a vertebra in front of the atlas and is called the pro-atlas; but as it is a membrane bone it is not properly a vertebral element.

The second or axis vertebra also differs a good deal from the other cervicals. The centrum is massive, and is terminated in front by a very large slightly concave articulating surface formed by the odontoid process (fig. 41, 3) which is united with the centrum by suture only, and is really the detached centrum of the first vertebra. The cervical rib (fig. 41, 9) articulates with two little irregularities on the odontoid process. The posterior surface of the centrum is convex. The neural arch is strongly developed and terminated dorsally by a long neural spine (fig. 41, 5), its sides are notched, slightly in front and more prominently behind for the exit of the spinal nerves. It is drawn out in front into two little processes bearing a pair of upwardly and outwardly directed prezygapophyses, while the postzygapophyses are similar to those of the other cervical vertebrae.

The last two cervical vertebrae resemble the succeeding thoracic vertebrae, in the increased length of the transverse processes, and the shifting dorsalwards of the facet with which the capitulum of the rib articulates.

The Thoracic vertebrae.

The thoracic vertebrae commence with the first of those that bears ribs reaching the sternum. They are ten in number, and the first eight are directly connected with the sternum by ribs.

The third of them may be taken as a type. It has a thick cylindrical centrum, concave in front and convex behind, there is a slight hypapophysis, and the centrum is suturally united with a strong neural arch enclosing a narrow neural canal. The neural arch is drawn out dorsally into a wide truncated neural spine, and laterally into two prominent transverse processes, with the ends of which the tubercula of the ribs articulate, while the capitulum articulates in each case with a step-like facet (fig. 42, A, 3) on the anterior face of the transverse process. The prezygapophyses (fig. 42, A, 2) are borne on outgrowths from the bases of the transverse processes, and the postzygapophyses on outgrowths at the base of the neural spine.

The thoracic vertebrae behind the third have no hypapophyses, and the capitular facets gradually come to be placed nearer and nearer the ends of the transverse processes, at the same time becoming less prominent; otherwise these vertebrae are just like the third.

Fig. 42. Anterior view of A, a late thoracic and B, the first sacral vertebra of a young Crocodile (C. palustris). × 1/3.

1. neural spine.	4. sacral rib.
2. process bearing	5. surface which is united with
prezygapophysis.	the ilium.
3. facet for articulation with the	6. concave anterior face of
capitulum of the rib.	centrum.

In the first and second thoracic vertebrae the capitulum of the rib articulates, not with a facet on the transverse process, but with a little elevation borne at the line of junction of the centrum and neural arch.

The Lumbar vertebrae.

These are five in number, and are precisely like the posterior thoracic vertebrae, except in the fact that the transverse processes have no facets for the articulation of ribs.

The Sacral vertebrae.

These are two in number, and while the centrum of the first is concave in front (fig. 42, B, 6) and nearly flat behind, that of the second is flat in front and concave behind. Each has a pair of strong ribs (fig. 42, B, 4) firmly ankylosed in the adult with a wide surface furnished partly by the centrum, partly by the neural arch. The distal ends of these ribs are united with the ilia. The character of the neural spines and zygapophyses is the same as in the thoracic vertebrae.

The Caudal vertebrae.

These are very numerous, about thirty-four in number. The first differs from all the other vertebrae of the body in having a biconvex centrum. The succeeding ones are procoelous and are very much like the posterior thoracic and lumbar vertebrae, having high neural spines and prominent straight transverse processes. They differ however in having the neural spines less strongly truncated above, and the transverse processes arise from the centra and not from the neural arches. When followed further back the centra and neural spines gradually lengthen while the transverse processes become reduced, and after the twelfth vertebra disappear. Further back still the neural spines and zygapophyses gradually become reduced and disappear, as finally the neural arch does also, so that the last few vertebrae consist simply of cylindrical centra.

Each caudal vertebra, except the first and the last eleven or so, has a V-shaped chevron bone attached to the postero-ventral edge of its centrum. The anterior ones are the largest and they gradually decrease in size till they disappear.

B. The Skull[90].

The skull of the Crocodile is a massive depressed structure presenting a number of striking characteristics, some of the more important of which are:—

1. All the bones except the mandible, hyoid, and columella are firmly united by interlocking sutures. In spite of this, however, growth of the whole skull and of the component bones goes on continuously throughout life, this growth being especially marked in the case of the facial as opposed to the cranial part of the skull.

2. All the bones appearing on the dorsal surface are remarkable for their curious roughened and pitted character; this feature is prominent also in many Labyrinthodonts.

3. The size of the jaws and teeth is very great.

4. The mandibular condyle is carried back to some distance behind the occipital condyle.

5. The occipital plane (see p. 386) of the skull is vertical.

6. The length of the secondary palate is remarkably great, and the vomer takes no part in its formation.

7. The posterior nares are placed very far back, the nasal passages being as in mammals separated from the mouth by the long secondary palate.

8. There is a complicated system of Eustachian passages communicating at one end with the tympanic cavity and at the other end with the mouth cavity.

9. The interorbital septum is mainly cartilaginous, the presphenoidal and orbitosphenoidal regions remaining unossified.

The skull is divisible into three parts:—

(1) the cranium, (2) the lower jaw, (3) the hyoid.

The cranium may again for purposes of description be divided into:—

1. the cranium proper or brain case;

2. the bones connected with the several special sense organs;

3. the bones of the upper jaw, and suspensorial apparatus.

1. The Cranium proper or brain case.

Fig. 43. Palatal aspect A, of the cranium, B, of the mandible of an Alligator (Caiman latirostris). × 1/3. (Brit. Mus.)

1. premaxillae.	12. quadratojugal.
2. maxillae.	13. quadrate.
3. palatine.	14. dentary.
4. pterygoid.	15. splenial.
5. posterior nares.	16. coronoid.
6. transpalatine.	17. supra-angular.
7. posterior palatine vacuity.	18. angular.
8. anterior palatine vacuity.	19. articular.
9. basi-occipital.	20. lateral temporal fossa.
10. opening of median	21. openings of vascular canals
Eustachian canal.	leading into alveolar sinus.
11. jugal.

The cartilage and membrane bones of the cranium proper when taken together can in most vertebrates be seen to be more or less arranged in three rings or segments called respectively the occipital, parietal and frontal segments; in the Crocodile however only the occipital and parietal segments are clearly seen.

The occipital segment consists of four cartilage bones, three of which together surround the foramen magnum.

The most ventral of these, the basi-occipital (figs. 43 and 45, 9), forms the single convex occipital condyle for articulation with the atlas, bounds the base of the foramen magnum, and is continuous laterally with two larger bones, the exoccipitals (fig. 45, 24), which meet one another dorsally and form the remainder of the boundary of the foramen magnum. Each is drawn out externally into a strong process, which is united below with the quadrate, and above with the squamosal by a surface seen in a disarticulated skull to be very rough and splintered. In a longitudinal section the anterior face of the exoccipital is seen to be closely united with the opisthotic.

The exoccipital is pierced by a number of foramina, four lying on the posterior surface. Just external to the foramen magnum is a small foramen for the exit of the hypoglossal nerve (figs. 44 and 45, XII). External to this is the foramen for the pneumogastric (fig. 44, X), while more ventrally still is the foramen (fig. 44, 15) through which the internal carotid artery enters the skull. Some distance further to the side, and more dorsally, is a larger foramen which gives passage to the facial nerve and certain blood-vessels.

In a median longitudinal section of the skull the hypoglossal foramen is seen, and just in front of it a small foramen for a vein. Further forwards the long slit-like opening between the exoccipital and opisthotic is the internal auditory meatus (fig. 45, VIII) through which the auditory nerve leaves the cranial cavity and enters the internal ear.

The supra-occipital (fig. 45, 5) is a small bone which takes no part in the formation of the foramen magnum, and is closely united in front with the epi-otic. It is characteristic of Crocodiles that all the bones of the occipital segment have their longer axes placed vertically, and that they scarcely if at all appear on the dorsal surface.

In front of the occipital segment is the parietal segment. The dorsal and ventral portions of the two segments are in contact with one another, but the lateral portions are widely separated by the interposition of the auditory and suspensorial bones.

The basisphenoid (fig. 45, 12) is an unpaired wedge-shaped bone, united along a deep vertical suture with the basi-occipital. The two bones are, however, partially separated in the mid-ventral line by a foramen, the opening of the median Eustachian canal, which leads into a complicated system of Eustachian passages ultimately communicating with the tympanic cavity.

The dorsal surface of the basisphenoid is well seen in a section of the skull, but owing to the way it tapers ventrally, it appears on the ventral surface only as a very narrow strip of bone wedged in between the basi-occipital and pterygoids. In a lateral view it is seen to be drawn out in front into an abruptly truncated process, the rostrum, which forms part of the interorbital septum. On the anterior part of the dorsal surface is a deep pit, the pituitary fossa or sella turcica, at the base of which are a pair of foramina, through which the carotid arteries pass. Dorsolaterally the basisphenoid articulates with the alisphenoids.

Fig. 44. Lateral view of the skull of an Alligator (Caiman latirostris). × 1/3. (Brit. Mus.)

1. premaxillae.	9. palatine.	16. external auditory	22. angular.
2. maxillae.	10. pterygoid.	meatus.	III, VI, opening for exit
3. lachrymal.	11. transpalatine.	17. frontal.	of oculomotor and
4. prefrontal.	12. quadratojugal.	18. supra-angular.	abducens nerves.
5. jugal.	13. exoccipital.	19. articular.	V, foramen ovale.
6. postfrontal.	14. basi-occipital.	20. dentary.	X, pneumogastric foramen.
7. squamosal.	15. foramen by which carotid	XII, hypoglossal foramen.
8. quadrate.	artery enters skull.	21. coronoid.

The alisphenoids (fig. 45, 13) are a pair of irregular bones which arise from the basisphenoid antero-laterally, and are united dorsally with the parietal, frontal, and postfrontals. They bound most of the anterior part of the brain case, and each presents on its inner face a deep concavity which lodges the cerebral hemisphere of its side. Viewed from the ventral side the two alisphenoids are seen to almost or quite meet one another immediately below the frontal, and then to diverge, forming an irregular opening—partially closed by cartilage in the fresh specimen,—through which the optic nerves leave the cranial cavity. Further back the alisphenoids meet one another for a narrow area, and then diverge again, so that between each and the rostrum of the basisphenoid there appears an opening (fig. 44, III, VI) through which the oculomotor and abducens nerves leave the cranium. Further back still each is united for a short space with the basisphenoid, pterygoid and quadrate, and then becomes separated from the quadrate by a large foramen, the foramen ovale (fig. 44, V), through which the whole of the trigeminal nerve passes out.

The dorsal portion of the parietal segment is formed by the parietal (fig. 45, 4), which though double in the embryo, early comes to form a single bone. It extends over the posterior part of the cranial cavity, and is continuous in front with the frontal, behind with the supra-occipital, and laterally with the postfrontals, squamosals, alisphenoids, pro-otics and epi-otics. It forms the inner boundary of a large rounded vacuity on the roof of the skull, the supratemporal fossa.

The frontal segment is very imperfectly ossified, there being no certain representatives of either the ventral member, the presphenoid, or the lateral members, the orbitosphenoids. On the dorsal side there is, however, a large development of membrane bones. There is a large frontal (fig. 45, 3), unpaired, except in the embryo, united behind with the parietal and postfrontal, and drawn out in front into a long process which is overlapped by the prefrontals and posterior part of the nasals. The frontal ends off freely below, owing to the orbitosphenoidal region being unossified, it forms a considerable part of the roof of the cranial cavity, but takes no part in the formation of the wall.

Each prefrontal (fig. 45, 14) forms part of the inner wall of the orbit and sends ventralwards a process which meets the palatine.

The postfrontals (fig. 44, 6) are small bones lying at the sides of the posterior part of the frontal. Each is united with a number of bones, on its inner side with the frontal and parietal, behind with the squamosal, and ventrally with the alisphenoid. It also unites by means of a strong descending process with an upgrowth from the jugal, and thus forms a postorbital bar separating the orbit from the lateral temporal fossa. The postfrontal forms also part of the outer boundary of the supratemporal fossa.

2. The Sense capsules.

Skeletal capsules occur in connection with each of the three special sense organs of sight, of hearing and of smell.

The Auditory capsules and associated bones.

Three bones, the epi-otic, opisthotic and pro-otic, together form the auditory or periotic capsule of each side. They are wedged in between the lateral portions of the occipital and parietal segments and complete the cranial wall in this region. Their relations to the surrounding structures are very complicated, and many points can be made out only in sections of the skull passing right through the periotic capsule. The relative position of the three bones is, however, well seen in a median longitudinal section. The opisthotic early becomes united with the exoccipital, while the epi-otic similarly becomes united with the supra-occipital, the pro-otic (fig. 45, 7),—seen in longitudinal section to be pierced by the prominent trigeminal foramen—alone remaining distinct throughout life. The three bones together surround the essential organ of hearing which communicates laterally with the deep tympanic cavity by the fenestra ovalis.

The tympanic cavity, leading to the exterior by the external auditory meatus (fig. 44, 16), is well seen in a side-view of the skull; it is bounded on its inner side by the periotic bones, posteriorly in part by the exoccipital, and elsewhere mainly by the quadrate. A large number of canals and passages open into it. On its inner side opening ventro-anteriorly is the fenestra ovalis, opening ventro-posteriorly the internal auditory meatus (fig. 45, VIII), while dorsally there is a wide opening which forms a communication through the roof of the brain-case with the tympanic cavity of the other side. On its posterior wall is the prominent foramen through which the facial nerve passes on its way to its final exit from the skull through the exoccipital, this foramen is bounded by the quadrate, squamosal, and exoccipital.

The opening of the fenestra ovalis is in the fresh skull occupied by the expanded end of the auditory ossicle, the columella, whose outer end articulates by a concave facet with a trifid extra-columellar cartilage which reaches the tympanic membrane. The lower process of this extra-columella passes into a cartilaginous rod which lies in a canal in the quadrate and is during life continuous with Meckel's cartilage within the articular bone of the mandible.

The columella and extra-columella are together homologous with the chain of mammalian auditory ossicles.

The Optic capsules and associated bones.

Two pairs of bones are associated with the optic capsules, viz. the lachrymals and the supra-orbitals. The lachrymal (fig. 44, 3) is a fairly large flattened bone lying wedged in between the maxillae, nasal, jugal, and prefrontal. It forms a considerable part of the anterior boundary of the orbit, and is pierced by two foramina. On the orbital edge is a large hole leading into a cavity within the bone which lodges the naso-lachrymal sac, and communicates with the narial passage by a wide second foramen near the anterior end of the bone. The supra-orbital is a very small loose bone lying in the eyelid close to the junction of the frontal and prefrontal.

The Olfactory capsules and associated bones.

Two pairs of membrane bones, the vomers and nasals, are developed in association with the olfactory organ, but the mesethmoid is not ossified.

The vomers form a pair of delicate bones, each consisting of a vertical plate (fig. 45, 15), which with its fellow separates the two narial passages, and of a horizontal plate which forms much of their roof. The vomers articulate with one another and with the pterygoids, palatines, and maxillae.

The nasals (fig. 45, 2) are very long narrow bones extending along the middle line from the frontal almost to the anterior nares. They are continuous laterally with the premaxillae, maxillae, lachrymals and prefrontals. They form the roof of the narial passages.

3. The Upper Jaw and suspensorial apparatus.

These are enormously developed in the Crocodile and are firmly united to the cranium. It will be most convenient to begin by describing the bones at the anterior end of the jaw and to work back thence towards the brain-case. The most anterior bones are the premaxillae. The premaxillae (figs. 44 and 45, 1) are small bones, each bearing five pairs of teeth, set in separate sockets in their alveolar borders. They constitute almost the whole of the boundary of the anterior nares, which are confluent with one another and form a large semicircular opening in the roof of the skull, leading into the wide narial passage. They are also partially separated from one another in the ventral middle line, by the small anterior palatine vacuity (fig. 43, A, 8). They form the anterior part of the broad palate. The alveolar border on each side between certain of the teeth is marked by pits which receive the points of the teeth of the other jaw. The first pair of these pits in the premaxillae are often so deep as to be converted into perforations. Pits of the same character occur between the maxillary and mandibular teeth.

Fig. 45. Longitudinal section through the skull of an Alligator (Caiman latirostris). × 1/3. (Brit. Mus.)

1. premaxillae.	for the trigeminal nerve.	16. maxillae.	25. squamosal.
2. nasal.	8. opisthotic.	17. palatine.	26. jugal.
3. frontal.	9. basi-occipital.	18. dentary.	27. external mandibular
4. parietal.	10. quadrate.	19. splenial.	foramen.
5. supra-occipital.	11. pterygoid.	20. angular.	28. internal mandibular
6. epi-otic.	12. basisphenoid.	21. supra-angular.	foramen.
7. pro-otic.	13. alisphenoid.	22. articular.	VIII. internal auditory
immediately in front of the	14. prefrontal.	23. coronoid.	meatus.
figure 7 is the prominent foramen	15. vomer.	24. exoccipital.	XII. hypoglossal foramen.

The maxillae (figs. 43, A, 2 and 44, 2) are a pair of very large bones and bear the remaining teeth of the upper jaw, set in sockets along their alveolar borders. On the dorsal side each maxillae is continuous with the premaxillae, nasal, lachrymal, and jugal, while ventrally it meets its fellow in a long straight suture and forms the greater part of the long bony palate. The maxillae are separated in the middle line posteriorly by processes from the palatines, while further back they meet the transpalatines. The internal or nasal surface, like that of the premaxillae, is excavated by a deep longitudinal groove, the narial passage. In a ventral view of the skull a number of small openings (fig. 43, A, 21) are seen close to the alveolar border, these are the openings of small vascular canals which lead into the alveolar sinus, a passage traversing the maxillae, and transmitting the superior maxillary branch of the trigeminal nerve and certain blood-vessels. This alveolar sinus opens posteriorly by the more external of the two large holes in the maxillae, which lie close to the anterior edge of the posterior palatine vacuity, to be described immediately. The more internal of these holes, on the other hand, leads into a cavity lodging the nasal sac. Behind the maxillae the completeness of the palate is broken up by the large oval posterior palatine vacuities (fig. 43, A, 7); these are separated from one another in the middle line by the palatines, and are bounded elsewhere by the maxillae, transpalatines, and pterygoids.

The palatines (fig. 43, A, 3) are long and rather narrow bones interposed between the maxillae in front and pterygoids behind. They meet one another in a long suture and form much of the posterior part of the palate, while the whole length of their dorsal surface contributes to the floor of the narial passage. The dorsal surface of each bone is also drawn out on its outer side into a prominent ridge which forms much of the side and roof of the narial passage, being in contact with the vomer and pterygoid, and at one point by means of a short ascending process with the descending process of the prefrontal.

The pterygoids (figs. 43, A, 4, and 45, 11) are a pair of large bones, each consisting of a median more or less vertical part, which becomes ankylosed to its fellow in the middle line early in life, and of a wide horizontal part which meets the transpalatine. They completely surround the posterior nares (fig. 43, A, 5) and their median portions form the whole boundary of the posterior part of the narial passage, and assist the palatines and vomers in bounding the middle part. The horizontal parts form the posterior part of the secondary palate, while the dorsal surface of each looks into the pterygoid fossa, a large cavity lying below the quadrate and quadratojugal at the side of the skull. The lateral margin adjoining the transpalatine is in the fresh skull terminated by a plate of cartilage against which the mandible plays. Dorsally the pterygoid articulates with the basisphenoid, quadrate, and alisphenoid.

The transpalatines (fig. 44, 11) connect the pterygoids with the jugals and maxillae, articulating with each of the three bones by a long pointed process. The jugal process meets also a down-growth from the postfrontal.

The jugals or malars (fig. 44, 5) are long somewhat flattened bones which are united to the lachrymals and maxillae in front, while passing backwards each is united behind to the quadratojugal (fig. 44, 12), the two forming the infratemporal arcade which constitutes the external boundary of the orbit and lateral temporal fossa. The jugal is united below to the transpalatine, and the two bones together form an outgrowth, which meeting that from the postfrontal forms the postorbital bar, and separates the orbit from the lateral temporal fossa. The quadratojugals are small bones and are united behind with the quadrates.

The quadrate (figs. 43, A, 13 and 44, 8) of each side is a large somewhat flattened bone firmly fixed in among the other bones of the skull. It is terminated posteriorly by an elongated slightly convex surface, coated with cartilage in the fresh skull, by which the mandible articulates with the cranium. The dorsal surface of the quadrate is flat behind, further forwards it becomes much roughened and articulates with the exoccipital and squamosal; further forwards still it becomes marked by a deep groove which forms the floor of the external auditory meatus and part of the tympanic cavity. The anterior boundary of the quadrate is extremely irregular, it is united dorsally with the postfrontal, pro-otic, and squamosal, and more ventrally with the alisphenoid. The smooth ventral surface looks into the pterygoid fossa. In front the quadrate forms the posterior boundary of the supratemporal fossa and foramen ovale, and is continuous with the alisphenoid, while it sends down a thin plate meeting the pterygoid and basisphenoid. On the inner side of the dorsal surface of the quadrate near the condyle, is a small foramen which leads into a tube communicating with the tympanic cavity, by a foramen lying in front of and ventral to that for the exit of the facial nerve. By this tube air can pass from the tympanic cavity into the articular bone of the mandible.

The squamosal (fig. 44, 7) meets the quadrate and exoccipital below, and forms part of the roof of the external auditory meatus, while above it forms part of the roof of the skull and has a pitted structure like that of the other bones of the roof. It is continuous with the postfrontal in front, forming with it the supratemporal arcade which constitutes the outer boundary of the supratemporal fossa. It meets also the parietal on its inner side, forming the post-temporal bar, the posterior boundary of the supratemporal fossa.

It may be useful to recapitulate the large vacuities in the surface of the Crocodile's cranium.

Dorsal surface.

1. The Supratemporal fossae. Each is bounded internally by the parietal, behind by the post-temporal bar formed by the parietal and squamosal, and externally by the supratemporal arcade formed by the squamosal and postfrontal. The postfrontal meets the parietal in front and forms the anterior boundary of the supratemporal fossa.

2. The Lateral temporal or infratemporal fossae. These lie below and to the outer side of the supratemporal fossae. Each is bounded dorso-internally by the supratemporal arcade; and behind by a continuation of the post-temporal bar formed by the quadrate and quadratojugal. The external boundary is the infratemporal arcade formed of the quadratojugal and jugal, while in front the fossa is separated from the orbit by the postorbital bar formed by the junction of outgrowths from the postfrontal and jugal.

3. The Orbits. Each is bounded behind by the postorbital bar, externally by the jugal forming a continuation of the infratemporal arcade, in front by the lachrymal, and internally by the frontal and prefrontal.

4. The Anterior nares. These form an unpaired opening bounded by the premaxillae.

Posterior surface.

5. The Foramen magnum. The exoccipitals form the chief part of its boundary, but part of the ventral boundary is formed by the basi-occipital.

6. The Pterygoid fossae. These form a pair of large cavities at the sides of the occipital region of the skull. The dorsal boundary is formed by the quadrate and quadratojugal, the ventral by the pterygoid, the internal chiefly by the quadrate, pterygoid, alisphenoid, and basisphenoid. The transpalatine forms a small part of the external boundary which is incomplete.

Ventral surface.

7. The Posterior nares. These form a median unpaired opening (fig. 43, A, 5) bounded by the pterygoids.

8. The Posterior palatine vacuities. Each is bounded by the maxillae in front, the maxillae and transpalatine externally, the transpalatine and pterygoid behind, and the palatine on the inner side (fig. 43, A, 7).

9. The Anterior palatine vacuity. This is unpaired and is bounded by the premaxillae (fig. 43, A, 8).

(b) The Lower Jaw or Mandible.

The mandible is a strong compact bony structure formed of two halves or rami, which are suturally united at the symphysis in the middle line in front. Each ramus is formed of six separate bones.

The most anterior and largest of these is the dentary (figs. 44, 20, and 45, 18), which forms the symphysis, and greater part of the anterior half of the jaw, and bears along the outer part of its dorsal border a number of sockets or alveoli in which the teeth are placed. Lying along the inner side of the dentary is a large splint-like bone, the splenial (fig. 45, 19), which does not extend so far forwards as the symphysis, and is separated from the dentary posteriorly by a large cavity. Forming the lower part of all the posterior half of the jaw is the large angular (figs. 44, 22, and 45, 20), which underlies the posterior part of the dentary in front and sends a long process below that bone to the splenial. On the inner side of the jaw there is an oval vacuity, the internal mandibular foramen (fig. 45, 28), between the angular and the splenial; through this pass blood-vessels and branches of the inferior dental nerve. Lying dorsal to the angular is another large bone, the supra-angular (figs. 44, 18, and 45, 21). It extends back as far as the posterior end of the jaw and forwards for some distance dorsal to the dentary and splenial. It forms part of the posterior margin of a large vacuity, the external mandibular foramen, which is bordered above and in front by the dentary and below by the angular; it gives passage to the cutaneous branch of the inferior dental nerve. The concave surface for articulation with the mandible and much of the posterior end of the jaw is formed by a short but solid bone, the articular (fig. 45, 22), which in young skulls rather readily becomes detached. The remaining mandibular bone is the coronoid (fig. 45, 23), a very small bone of irregular shape attached to the angular below, and to the supra-angular and splenial above.

(c) The Hyoid.

The hyoid of the Crocodile consists of a wide flattened plate of cartilage, the basilingual plate or body of the hyoid, and a pair of cornua.

The basilingual plate (fig. 53, 1) is rounded anteriorly and marked by a deep notch posteriorly. The cornua (fig. 53, 3), which are attached at a pair of notches near the middle of the outer border of the basilingual plate, are partly ossified, but their expanded ends are formed of cartilage. They pass at first backwards and then upwards and inwards. They are homologous with part of the first branchial arches of Selachians.

The columella and extra-columella have been already described (p. 251).

C. The Ribs and Sternum.

Thoracic ribs.

The Crocodile has ten pairs of thoracic ribs, all except the last one or two of which consist of three parts,—a vertebral rib, an intermediate rib and a sternal rib.

Of the vertebral ribs the third may be taken as a type, it consists of a curved bony rod which articulates proximally with the transverse process of the vertebra by two facets. The terminal one of these, the capitulum or head, articulates with a notch on the side of the transverse process; the other, the tuberculum, which lies on the dorsal surface a short distance behind the head, articulates with the end of the transverse process. From near the distal end an imperfectly ossified uncinate process (see p. 190) projects backwards.

The intermediate ribs are short and imperfectly ossified; they are united with the sternal ribs (fig. 46, 3), which are large, flattened, likewise imperfectly ossified structures, and articulate at their distal ends with a pair of long divergent xiphisternal horns (fig. 46, 5), which arise from the posterior end of the sternum proper. The last pair of sternal ribs are attached to the preceding pair, not to the xiphisternal horns.

The first and second vertebral ribs differ from the others in the fact that the tuberculum forms a fairly long outstanding process.

Cervical ribs.

Movable ribs are attached to all the cervical as well as to the thoracic vertebrae. Those borne by the atlas and axis are long, narrow structures attached by a fairly broad base, and tapering gradually. The ribs borne by the third to seventh cervical vertebrae are shaped like a T with a double base, one limb of which, corresponding to the tuberculum (fig. 41, 7), articulates with a short transverse process arising from the neural arch, while the other, corresponding to the capitulum, articulates with a surface on the centrum. The ribs attached to the eighth and ninth cervical vertebrae are intermediate in character between the T-shaped ribs and the ordinary thoracic ribs. The anterior limb of the T is shortened, the posterior one is drawn out, forming the shaft of the rib. The distal portion of the rib of the ninth cervical vertebra is unossified.

The Sacral ribs have been described in connection with the sacral vertebrae.

The Sternum.

The sternum of Crocodiles is a very simple structure, consisting of a plate of cartilage (fig. 46, 2) lying immediately dorsal to the interclavicle, and drawn out posteriorly into a pair of long xiphisternal horns (fig. 46, 5).

Fig. 46. Sternum and Associated Membrane bones of a Crocodile (C. palustris) × 1/3. (Brit. Mus.)
The last pair of abdominal ribs which are united with the epipubes by
a plate of cartilage have been omitted.

1. interclavicle.	4. abdominal splint rib.
2. sternum.	5. xiphisternal horn.
3. sternal rib.

The abdominal splint ribs.

Lying superficially to the recti muscles of the ventral body-wall, behind the sternal ribs, are seven or eight series of slender curved bones, the abdominal ribs (fig. 46, 4). Each series consists of four or more bones, arranged in a V-like form with the angle of the V directed forwards. They show a considerable amount of variability in number and character. They are really membrane bones, and are in no way homologous with true ribs, but correspond rather with the more posterior of the bones constituting the plastron of Chelonia.

2. The Appendicular Skeleton.

This includes the skeleton of the two pairs of limbs and their respective girdles.

Fig. 47. Left half of the pectoral girdle of an Alligator (Caiman latirostris) × 2/3. (Brit. Mus.)

1. scapula.	3. interclavicle.
2. coracoid.	4. glenoid cavity.

The Pectoral girdle.

The pectoral girdle of the Crocodile is less complete than is that of most reptiles. It consists of a dorsal bone, the scapula, and a ventral bone, the coracoid, with a median unpaired element, the interclavicle; but there is no separate representative either of the clavicle or precoracoid.

The scapula (fig. 47, 1) is a large bone, flattened and expanded above where it is terminated by an unossified margin, the suprascapula, and thickened below where it meets the coracoid. The scapula forms about half the glenoid cavity (fig. 47, 4) for articulation with the humerus, and has the lower part of its anterior border drawn out into a roughened ridge.

The coracoid (fig. 47, 2) is a flattened bone, much expanded at either end; it bears on its upper posterior border a flattened surface which forms half the glenoid cavity, and is firmly united to the scapula at its dorsal end. Its ventral end meets the sternum.

The interclavicle (figs. 46, 1, and 47, 3) is a long narrow blade-shaped bone lying along the ventral side of the sternum; about a third of its length projects beyond the sternum in front.

The Anterior limb.

This is as usual divisible into three portions, the upper arm, fore-arm and manus.

The upper arm or brachium contains one bone, the humerus.

The humerus (fig. 48, A, 1) is a fairly long stout bone, considerably expanded at either end. The proximal end or head is evenly rounded and is formed by an epiphysis ossifying from a centre different from that forming the shaft. It articulates with the glenoid cavity. The shaft bears on the flexor surface, at some little distance behind the head, a prominent rounded protuberance, the deltoid ridge. The distal end or trochlea is also formed by an epiphysis and is partially divided by a groove into two convex surfaces; it articulates with the two bones of the fore-arm, the radius and ulna.

Fig. 48. A, right anterior, and B, right posterior limb of a young Alligator (Caiman latirostris). (Brit. Mus.)
A × ½. B × about 1/3.

1. humerus.	10. proximal phalanx of second
2. radius.	digit.
3. ulna.	11. second phalanx of fifth digit.
4. radiale.	12. femur.
5. ulnare.	13. tibia.
6. pisiform.	14. fibula.
7. patch of cartilage representing	15. tibiale, intermedium and
carpalia 1 and 2; between	centrale (fused).
it and the radiale should be	16. fibulare.
another flattened patch, the	17. tarsalia 1, 2, and 3 (fused).
centrale.	18. tarsalia 4 and 5 (fused).
8. carpalia 3, 4, and 5 (fused).	19. first metatarsal.
9. first metacarpal.	20. ungual phalanx of second digit.
	21. fifth metatarsal.

The radius and ulna are nearly equal in size and each consists of a long shaft terminated at either end by an epiphysis.

The radius (fig. 48, A, 2) or pre-axial bone is slightly the smaller of the two. It has a straight cylindrical shaft and is slightly and nearly evenly expanded at either end. The proximal end which articulates with the humerus is flat or slightly concave, the distal end which articulates with the carpus is slightly convex.

The ulna (fig. 48, A, 3) or postaxial bone is a curved bone rather larger than the radius. Its proximal end is large and convex, but is not drawn out into an olecranon process.

The Manus consists of the carpus or wrist, and the hand.

The Carpus. This differs considerably from the more primitive type met with in the Turtle. It consists of six elements arranged in a proximal row of three and a distal row of two, with one intervening. The bones of the proximal row are the radiale, the ulnare, and the pisiform. The radiale (fig. 48, A, 4) is the largest bone of the carpus: it is a somewhat hour-glass shaped bone, with its ends formed by flattened epiphyses. It articulates by its proximal end with the whole of the radius, and partly also with the ulna, and by its distal end with the centrale.

The ulnare (fig. 48, A, 5) is a smaller bone, also somewhat hour-glass shaped; it articulates proximally with the pisiform and radiale, not quite reaching the ulna. The third bone of the proximal row is the pisiform (fig. 48, A, 6), an irregular bone, articulating with the ulna, radiale, and fifth metacarpal. The centrale is a flattened cartilaginous element applied to the distal surface of the radiale.

The distal row of carpals consists of two small structures. The first of these forms a small cartilaginous patch, which is wedged in between the first and second metacarpals, the centrale and the bone representing carpalia 3, 4 and 5; this cartilaginous patch represents carpalia 1 and 2 (fig. 48, A, 7). The bone representing carpalia 3, 4 and 5 is a good deal larger, rounded, and well-ossified; it articulates with the ulnare, the pisiform, and the third, fourth, and fifth metacarpals.

The hand. Each of the five digits consists of an elongated metacarpal, terminated at each end by an epiphysis, and of a varying number of phalanges. The terminal phalanx of each digit has an epiphysis only at its proximal end, the others have them at both ends.

The first digit, or pollex, is the stoutest, and has two phalanges, the second has three, the third four, the fourth three, and the fifth two. The terminal phalanx of each of the first three digits is pointed and sheathed in a horny claw; and is also marked by a pair of prominent lateral grooves.

The Pelvic Girdle.

The pelvic girdle of the Crocodile consists of four parts, a dorsal element, the ilium, an anterior ventral element, the pubis, a posterior ventral element, the ischium, and an accessory anterior ventral element, the epipubis. All except the epipubis take part in the formation of the acetabulum, which is perforated by a prominent hole.

The ilium (fig. 49, 1) is a thick strong bone, firmly united on its inner side with the two sacral ribs. Its dorsal border is rounded, its ventral border bears posteriorly two irregular surfaces, completed by epiphyses, which are united respectively with the ischium and pubis.

The ischium (fig. 49, 2)—the largest bone of the pelvis, is somewhat contracted in the middle and expanded at either end. Its proximal end, which is formed by an epiphysis, bears two surfaces, one of which is united to the ilium, while the other forms part of the acetabulum. The anterior border is also drawn out dorsally into a strong process, which is terminated by a convex epiphysis, and is united to the pubis. The ventral end of the ischium forms a flattened blade, meeting its fellow in a median symphysis.

The pubis (fig. 49, 3) is much smaller than either the ilium or ischium; it forms a small patch of unossified cartilage, interposed between the anterior parts of the ilium and ischium.

Fig. 49. Pelvis and sacrum of an Alligator (Caiman latirostris) × ½.
(Brit. Mus.)

1. ilium.	6. neural spines of sacral
2. ischium.	vertebrae.
3. true pubis.	7. symphysis ischii.
4. epipubis (so-called pubis).	8. process bearing
5. acetabular foramen.	prezygapophysis.

The epipubis (fig. 49, 4) is a large bone with a thickened proximal end, which is loosely articulated to the ischium, and a flattened expanded distal end, which is united with its fellow, and with the last pair of abdominal ribs by a large plate of cartilage. This bone is generally described as the pubis.

The Posterior limb.

This is as usual divisible into three portions, the thigh, the crus or shin, and the pes.

The thigh is formed by the femur (fig. 48, B, 12), a moderately long stout bone, not unlike the humerus; it articulates with the acetabulum by a fairly prominent rounded head. The distal end articulating with the tibia and fibula is also expanded, and is partially divided into equal parts by anterior and posterior grooves. The flexor surface bears a fairly prominent trochanteric ridge. Each end of the femur is formed by an epiphysis.

The crus or shin includes two bones, the tibia and fibula. Both are well developed, but the tibia is considerably the larger of the two.

The tibia (fig. 48, B, 13) is a strong bone with a flattened expanded proximal end articulating with almost the whole of the end of the femur, and a similarly expanded distal end articulating with a bone representing the fused astragalus and centrale.

The fibula (fig. 48, B, 14) is flattened proximally, and articulates with only quite a small part of the femur, while distally it is more expanded, and articulates with the fibulare (calcaneum) and with a facet on the side of the fused astragalus and centrale.

The Pes consists of the tarsus or ankle, and the foot.

The Tarsus. This, like the carpus, is much reduced and modified from the primitive condition. It consists of only four bones, arranged in two rows of two each. The two bones of the proximal row are much larger than are those of the distal row. The pre-axial of them (fig. 48, B, 15) representing the fused astragalus (tibiale and intermedium) and centrale, articulates proximally with the tibia and fibula, and distally with the first metatarsal, and a small bone representing the first three tarsalia. The postaxial bone, the calcaneum (fibulare) (fig. 48, B, 16), is drawn out into a prominent posterior process forming a heel such as is almost unknown elsewhere except in mammals. It articulates with the fibula, the tibiale-centrale, and distally with a bone representing the fourth and fifth tarsalia, and with the fifth metatarsal.

The two bones forming the distal row of tarsals are both small and rounded; one represents the first three tarsalia fused together, the other tarsalia 4 and 5.

The Foot. The foot has five digits, but the fifth is much reduced, consisting only of a short metatarsal. The first four metatarsals are all long bones, slightly expanded at each end, and terminated by small epiphyses. The first digit has two phalanges, the second three, the third four, and the fourth five. The terminal or ungual phalanx in each instance is grooved and pointed, and in the case of the first three digits bears a horny claw. The ungual phalanx progressively decreases in size from the first to the fourth. The fifth digit consists only of a small, somewhat square metatarsal (fig. 48, B, 21), attached to the bone representing the fused fourth and fifth tarsalia.

EXOSKELETON.

The exoskeleton both epidermal and dermal is exceedingly well developed in reptiles.

Epidermal Exoskeleton.

This generally has the form of overlapping horny scales which invest outgrowths of the dermis, and are found covering the whole body in most Rhynchocephalia, Ophidia, and Lacertilia, and many Crocodilia. In the Ophidia the ventral surface of the tail is commonly covered by a double row of broad scales, while the ventral surface of the precaudal part of the body is covered by a single row. In the burrowing snakes (Typhlopidae) and some sea snakes (Hydrophidae) these broad scales do not occur, the scales of the ventral surface being similar to those of the dorsal.

In the Chelonia with the exception of Dermochelys, Trionyx and their allies there is a well-developed system of horny shields having a regular arrangement which has been described in the account of the Turtle's skeleton[91].

The rattle of the rattlesnake is an epidermal structure formed of several loosely articulated horny rings, produced by the modification of the epidermal covering of the end of the tail, which instead of being cast off when the rest of the outer skin is shed is retained loosely interlocked with the adjoining ring or joint. New rings are thus periodically added to the base of the rattle, and in old animals the terminal ones wear away and are lost.

Horny claws occur on the ends of some or all of the digits in most living reptiles.

Owen's Chameleon bears three epidermal horns, one arising from the nasal and two from the frontal region.

In the Chelonia, some of the Theromorpha such as Udenodon and Dicynodon, probably also in the Pterosauria and Polyonax among the Dinosaurs, the jaws are more or less cased in horny beaks. The horny beaks of Chelonia are variable; sometimes they have cutting edges, sometimes they are denticulated, sometimes they are adapted for crushing.

Dermal Exoskeleton.

Nearly all Crocodilia, many Dinosauria, some Rhynchocephalia and Pythonomorpha, and some Lacertilia such as Tiliqua, Scincus and Anguis have a dermal exoskeleton of bony scutes, developed below and corresponding in shape to the epidermal scales. Sometimes as in Caiman sclerops, Jacare and Teleosaurus, the scutes completely invest the body, being so arranged as to form a dorsal and a ventral shield, and a continuous series of rings round the tail. In Crocodilus they are confined to the dorsal surface, and in Alligator to the dorsal and ventral surfaces. The scutes of some extinct forms articulate with one another by a peg and socket arrangement as in some Ganoid fish.

The carapace of most Chelonia is a compound structure, being partly endoskeletal and formed from the ribs and vertebrae, partly from plates derived from the dermal exoskeleton. The common arrangement is seen in fig. 36. All the surface plates are probably exoskeletal in origin, but united with the ventral surfaces of the costal and neural plates respectively are the expanded ribs and neural arches of the vertebrae.

The plastron in the common genus Chelone (fig. 37) includes nine plates of bone, one unpaired and four pairs; they will be referred to in connection with the ribs and pectoral girdle.

In the Leathery Turtle (Dermochelys) the carapace and plastron differ completely from those of any other living form. The carapace consists of a number of polygonal ossifications fitting closely together and altogether distinct from the vertebrae and ribs. The plastron is imperfectly ossified, and not united with the pelvis, and the whole surface of both carapace and plastron is covered with a tough leathery skin, without horny shields.

Some of the extinct Dinosauria have an enormously developed dermal exoskeleton. Thus in Stegosaurus and Omosaurus the dorsal surface is provided with flattened plates or with spines reaching a length of upwards of two feet. In Polacanthus the posterior part of the body is protected by a bony shield somewhat recalling that of the little armadillo Chlamydophorus. No exoskeleton is known in Ichthyosauria, Sauropterygia, Pterosauria, many Dinosauria and Theromorpha, and some Lacertilia, such as Chamaeleon and Amphisbaena.

Teeth.

The teeth of reptiles are generally well developed, and in the great majority of forms are simple conical structures, uniform in character, generally somewhat recurved, and often with serrated edges. Another common type of tooth is that with a laterally compressed triangular crown provided with a double cutting edge which may or may not be serrated. The teeth are mainly formed of dentine, with usually an external layer of enamel, and often a coating of cement on the root. Vasodentine is found below the dentine in Iguanodon. The teeth of reptiles never have the enamel deeply infolded, nor do they have double roots.

Teeth may be present not only on the jaw-bones, but as in many Squamata, also on the palatines, pterygoids or vomers. The method by which they are attached to the bones varies much. Sometimes as in Iguana and some other lizards, they are pleurodont[1], sometimes they are acrodont[92], as in the Rhynchocephalia, Pythonomorpha, Ophidia and some Lacertilia such as Agama. Again they may be set in a continuous groove as in the Ichthyosauria and young Crocodilia. Finally the teeth may be thecodont or placed in distinct sockets as in the Theromorpha, Sauropterygia, adult Crocodilia, Sauropoda and Theropoda. In Iguanodon the teeth are set in shallow sockets in a groove one side of which is higher than the other; the method of attachment thus shows points of resemblance to the thecodont condition, the pleurodont condition, and that met with in the Ichthyosauria.

In Ichthyosaurus the teeth are marked by a number of vertical furrows, and it is from a furrow of this nature greatly enlarged and converted into a tube that the channel down which flows the poison of venomous snakes is derived.

In most reptiles the dentition is more or less homodont. The only reptiles in which a definite heterodont dentition is known are the extinct Theromorpha, and in them the teeth vary greatly. Thus Udenodon is toothless, the jaws having been probably cased in a horny beak. In Dicynodon the jaws are likewise toothless with the exception of a pair of permanently growing tusks borne by the maxillae. Dicynodon is the only known reptile whose teeth have permanently growing pulps. In Pariasaurus the teeth are uniform and very numerous, and though placed in distinct sockets are ankylosed to the jaw. In Galesaurus and Cynognathus three kinds of teeth can be distinguished, slender conical incisor-like teeth, large canine-like teeth, and cheek teeth with two or three cusps. The teeth in Galesaurus are confined to the jaws, in Placodus and its allies, however, large flat crushing teeth are attached to the palatines as well as to the jaw-bones, and in Pariasaurus the vomer, palatine and pterygoid all bear teeth as well as the jaw bones. The upper jaw of Sphenodon and other Rhynchocephalia is provided with two parallel rows of teeth, one borne on the maxillae and one on the palatines, the mandibular teeth bite in a groove between these two rows. The bone of the jaws in Sphenodon is so hard that when the teeth get worn away, it can act as a substitute. In the young Sphenodon the vomers bear teeth, as they do also in Proterosaurus.

There is generally a continuous succession of teeth throughout life, the new tooth coming up below, or partly at the side of the one in use, and causing the absorption of part of its wall or base. In this way the new tooth comes to lie in the pulp cavity of the old one. This method of succession is well seen in the Crocodilia.

Fig. 50. Preparation of part of the right mandibular ramus of Crocodilus palustris × ½. (Brit. Mus.)

1. tooth in use.	3. symphysial surface of the
2. fairly old germ of future	mandible.
tooth.

Teeth have been detected in embryos of Trionyx, but otherwise no teeth are known to occur in Chelonia, or in Pteranodon (Pterosauria), while the anterior part of the jaw is edentulous in Iguanodon, Polyonax and some other Dinosaurs, and in Rhamphorhynchus.

ENDOSKELETON.

Vertebral column.

The vertebral column is commonly divisible into the usual five regions, but in the Ophidia, Ichthyosauria, and Amphisbaenidae among Lacertilia, only into caudal and precaudal regions. In the Chelonia there are no lumbar vertebrae.

The form of the vertebral centra is very variable. A large proportion of extinct reptiles,—several entire orders,—and the earlier and more primitive forms in some of the other groups have amphicoelous vertebrae. Vertebrae of this type occur in the Theromorpha, Ichthyosauria, most Sauropterygia and Rhynchocephalia, and many Dinosauria, also in some of the early Crocodilia such as Belodon, Teleosaurus and Goniopholis, and the Geckonidae among Lacertilia.

The majority of living reptiles have procoelous vertebrae. Thus they occur in the Lacertilia (excluding the Geckos), the Ophidia, and the Crocodilia, also among extinct forms in the Pterosauria and many Dinosauria. On the other hand some Dinosauria such as Iguanodon have opisthocoelous cervical vertebrae, while others have opisthocoelous thoracic vertebrae. The vertebrae of the Ceratopsidae and some Sauropterygia, the thoracic vertebrae of Iguanodon, and the sacral vertebrae of Crocodilia have flat centra. The first caudal vertebra of modern Crocodilia is biconvex, and in the Chelonia all types of vertebral centra are found. The cervical vertebrae of Sphenodon are noticeable for the occurrence of a small pro-atlas, which may represent the neural arch of a vertebra in front of the atlas.

In most reptiles the vertebrae are fully ossified, but in some of the more primitive forms the notochord persists in the centre of the vertebra (i.e. intervertebrally), this is the case for instance in many of the Theromorpha and Rhynchocephalia, and also in the Geckos. In other reptiles it persists longest intravertebrally.

The centrum of each of the caudal vertebrae of most Lacertilia is traversed by an unossified septum along which it readily breaks.

Chevron bones occur below the caudal vertebrae in Lacertilia, Chelonia, Ichthyosauria, many Dinosauria, and Sphenodon, articulating with quite the posterior part of the centrum which bears them. In Lacertilia and Crocodilia (fig. 41, 3) the axis has a well-marked odontoid process. The ventral portions of the intervertebral discs are sometimes ossified, forming wedge-shaped inter centra, as in Geckos, and the cervical vertebrae of Sphenodon.

In snakes, Theropod Dinosaurs, and the iguanas among lizards, the neural arches are provided with zygosphenes, and zygantra.

The neural arches are usually firmly ankylosed to the centra, but in the Crocodilia and some Chelonia, Sauropterygia, and Dinosauria, the suture between the centrum and neural arch persists at any rate till late in life. In the Ichthyosauria the neural arches were united to the centra by cartilage only.

The thoracic vertebrae of some of the Theromorpha (Dimetrodon) are remarkable for the extraordinary development of the neural spine, and those of Chelonia for the absence of transverse processes.

In living reptiles the number of sacral vertebrae is nearly always two, but in the Theromorpha, Dinosauria, and Pterosauria, as many as five or six bones may be ankylosed together in the sacral region. In Crocodiles the two halves of the pelvis sometimes articulate with different vertebrae. The vertebrae of some of the great Sauropoda are remarkably hollowed out, having a large vacuity on each side of the centrum communicating with a series of internal cavities. The whole structure of these vertebrae shows a combination of great strength with lightness.

The Skull.

The reptilian skull is well ossified and the bones are noticeable for their density. The true cranium is often largely concealed by a secondary or false roof of membrane bones, which is best seen in the Ichthyosauria and some of the Chelonia. In other reptiles the false roof is more or less broken up by vacuities exposing the true cranial walls. The ethmoidal region is the only one in which much of the primordial cartilaginous cranium remains. The lateral parts of the sphenoidal region are also as a rule not well ossified.

In some reptiles, such as most Lacertilia and Chelonia, the orbits are separated only by the imperfect interorbital septum, while in others, such as the Ophidia, Crocodilia and Amphisbaenidae, the cranial cavity extends forwards between the orbits.

In the occipital region all four bones are ossified. The great majority of reptiles have a single convex occipital condyle, but some of the Theromorpha such as Cynognathus have two distinct condyles as in mammals. Sometimes, as in Chelonia, Ophidia and Lacertilia, the exoccipitals, as well as the basi-occipital, take part in the formation of the single condyle; sometimes, as in Crocodiles, it is formed by the basi-occipital alone, as in birds. The relations of the bones to the foramen magnum vary considerably, in Chelonia the basi-occipital generally takes no part in bounding it, and in the Theromorpha, Crocodilia, and Ophidia, the supra-occipital is excluded. The parietals are paired in Geckos and Chelonia alone among living forms, and in the extinct Ichthyosauria and some Theromorpha; in all other reptiles they are united.

The frontals are paired in Ichthyosauria (fig. 32, 5), Chelonia, Ophidia, Sphenodon (fig. 52, B, 4) and some extinct crocodiles, such as Belodon. They are completely fused in living Crocodilia and some Lacertilia and Dinosauria. In the gigantic Polyonax they are drawn out into a pair of enormous horns, and the parietals and squamosals are greatly expanded behind.

An interparietal foramen occurs in the Theromorpha, the Ichthyosauria (fig. 32, 10), Sphenodon, the Sauropterygia and most Lacertilia. The posterior part of the skull is curiously modified in some Chamaeleons, the parietals and supra-occipitals being drawn out into a backwardly-projecting sagittal crest which unites with the two prolongations from the squamosals. In other Chamaeleons (C. bifidus) prolongations of the prefrontals and maxillae form large forwardly-projecting bony processes.

The roof of the skull is characterised by the development of prefrontals and postfrontals, which lie respectively near the anterior and posterior extremity of the orbit. In Theromorpha, Squamata, Crocodilia, and some Dinosauria lachrymals are developed. There is a ring of bones in the sclerotic in the Ichthyosauria (fig. 32, 15), the Metriorhynchidae among Crocodiles and some Rhynchocephalia, Dinosauria, and Pterosauria.

The pro-otic lies in front of the exoccipital and together with the opisthotic forms the hind border of the fenestra ovalis. In Chelonia the opisthotic remains separate, in all other living reptiles it fuses with the exoccipital. The epi-otic fuses with the supra-occipital.

The parasphenoid, so important in Ichthyopsids, has very often disappeared completely; it is present, however, in the Ichthyosauria, the Plesiosauridae, and a number of Squamata, in many Ophidia its anterior part forming the base of the interorbital septum.

In the Plesiosauridae and most Lacertilia, but not in the Amphisbaenidae, a slender bone, the epipterygoid, occurs uniting the parietal or the anterior end of the pro-otic with the pterygoid. A homologous arrangement occurs in the Ichthyosauria and some Chelonia.

In most reptiles a transpalatine occurs, connecting the maxillae with the pterygoid, but this is absent in the Chelonia, and some Dinosauria, and in the Typhlopidae among snakes.

The quadrate is always well developed, and except in the Squamata is firmly fixed to the surrounding bones. The Chamaeleons also, among the Squamata, have a fixed quadrate, and in them too the quadratojugal is absent. Separate nasal bones do not occur in any living Chelonia.

The vomers are generally paired as in Squamata, sometimes unpaired as in Chelonia.

Fig. 51. Dorsal (to the left) and ventral (to the right) views of the skull of the Common Snake (Tropidonotus natrix). (After Parker.)

1. premaxillae (fused).	16. quadrate.
2. anterior nares.	17. parasphenoid.
3. nasal.	18. basisphenoid.
4. prefrontal.	19. basi-occipital.
5. frontal.	20. occipital condyle.
6. parietal.	21. splenial.
7. maxillae.	22. dentary.
8. transpalatine.	23. angular.
9. palatine.	24. articular.
10. pterygoid.	25. supra-angular.
11. pro-otic.	26. coronoid.
12. exoccipital.	27. vomer.
13. supra-occipital.	28. squamosal.
14. opisthotic.	IX, X foramina for the ninth
15. epi-otic.	and tenth cranial nerves.

The disposition of the bones of the jaws is subject to much modification in the Ophidia in order to adapt them for swallowing very large prey. The arrangements again differ greatly in the venomous and non-venomous snakes. In the non-venomous snakes, such as Python and Tropidonotus, the palatine is large and is fixed to the pterygoid which extends outwards (fig. 51, 10) so as to be united to the quadrate, and is at the same time firmly connected by the transpalatine with the maxillae. The quadrate is united to the squamosal, which is loosely attached to the cranium. The premaxillae is moderately developed and bears teeth, and the maxillae forms a long bar loosely connected with the rest of the skull. The rami of the mandible are united only by an extremely elastic ligament. It is as regards the maxillae and premaxillae that the skulls of venomous and non-venomous snakes differ most. In the rattlesnake (Crotalus) and other venomous snakes the premaxillae is extremely small and toothless. The maxillae is small and subcylindrical, and is movably articulated to the lachrymal, which also is capable of a certain amount of motion on the frontal. The maxillae is connected by means of the transpalatine with the pterygoid, which in its turn is united to the quadrate. When the mouth is shut the quadrate is directed backwards, and carrying back the pterygoid and transpalatine pulls at the maxillae and causes its palatal face, to which the poison teeth are attached, to lie back along the roof of the mouth. When the mouth opens the distal end of the quadrate is thrust forward, and this necessitates the pushing forward of the pterygoid and transpalatine, causing the tooth-bearing surface of the maxillae to look downwards and the tooth to come into the position for striking.

The Ophidian skull is also noticeable for the absence of the jugals and quadratojugals. In poisonous snakes the place of the jugal is taken by the zygomatic ligament which connects the quadrate and maxillae.

The extent to which the palate is closed in reptiles varies much. In many reptiles, such as the Squamata and Ichthyosauria, the palate is not complete, both palatines and pterygoids being widely separated in the middle line. In others, such as the Crocodilia, Sauropterygia, and Chelonia, there is a more or less complete bony palate. In many Chelonia this is chiefly formed of the vomer, palatines, and pterygoids, the posterior nares being mainly bounded by the palatines. In living Crocodilia, however, outgrowths are formed from the pterygoids and palatines which arch round and meet one another ventrally, forming a secondary palate (fig. 43, A), which completely shuts off the true sphenoidal floor of the skull, and causes the posterior nares which are bounded by the pterygoids to open very far back. Though this feature is common to all postsecondary crocodiles, it is interesting to notice that it is not found in the earlier forms, but that its gradual evolution can be traced. In the Triassic Belodon, for instance, the posterior nares open far forwards, and are not surrounded by either the palatines or pterygoids. In the Jurassic crocodile, Teleosaurus, the posterior nares lie further back, being surrounded by the palatines, but the pterygoids do not meet them. Finally, in the Tertiary forms the arrangements are as in living crocodiles.

A short secondary hard palate is found also in the Theriodontia. The palatines of Ichthyosaurus are noticeable for their transverse position, which recalls that in the Frog.

The various fossae or vacuities in the false roof of the skull are important, and their relations may best be understood by a description of their mode of occurrence in Sphenodon, a form in which they are very completely developed.

In Sphenodon, then, on the dorsal surface of the skull, are the large supratemporal fossae (fig. 52, 20). Their inner margins are separated from one another by the parietal walls of the cranium, while externally each is bounded by a bony arch, the supratemporal arcade, formed of the postfrontal, postorbital, and squamosal. Posteriorly the boundary is formed by a post-temporal bar, formed by the parietal and squamosal. Below the supratemporal arcade is another large vacuity, the infratemporal or lateral temporal fossa (fig. 52, 21). This is bounded above by the supratemporal arcade, and is separated from the orbit in front by the postorbital bar, formed by the union of outgrowths from the jugals and postorbitals. Behind it is bounded by a continuation of the post-temporal bar formed of the squamosal and quadratojugal, and below by an infratemporal arcade, which is chiefly composed of the quadratojugal and jugal.

Fig. 52. Skull of Hatteria. (Sphenodon punctatus). A, lateral; B, dorsal; C, ventral; D, posterior. (After von Zittel.)

1. premaxillae.	16. transpalatine.
2. nasal.	17. exoccipital.
3. prefrontal.	18. epipterygoid.
4. frontal.	19. basisphenoid.
5. postfrontal.	20. supratemporal fossa.
6. parietal.	21. infratemporal or lateral temporal
7. squamosal.	fossa.
8. quadratojugal.	22. orbit.
9. quadrate.	23. post-temporal fossa.
10. postorbital.	24. foramen magnum.
11. jugal.	25. anterior nares.
12. maxillae.	26. interparietal foramen.
13. vomer.	27. dentary.
14. palatine.	28. supra-angular.
15. pterygoid.	29. articular.

Below the post-temporal bar is a third vacuity, the post-temporal fossa (fig. 52, D, 23), bounded above by the post-temporal bar and below by the exoccipital and opisthotic.

Sphenodon and the Crocodilia are the only living reptiles with complete supratemporal and infratemporal arcades, but they are both present in the extinct Pterosauria and some Dinosauria.

Supratemporal fossae, bounded below by supratemporal arcades, occur in all reptiles except some Chelonia, the Ophidia, the Geckonidae among Lacertilia, and the Pariasauria and others among Theromorpha; they are specially large in Nothosaurus among the Sauropterygia, Dicynodon among the Theromorpha, and many Crocodilia and Pterosauria. In some Dinosaurs, such as Ceratosaurus, they are very small, while the infratemporal fossae are correspondingly large.

In Elginia[93] (Theromorpha) and some Chelonia, such as Chelone, there are no fossae on the surface of the skull, a complete false roof being developed; in other Chelonia, such as Trionyx, the true cranium is freely visible, the only part of the false roof developed being the infratemporal arcade.