The bony framework, or skeleton, that which gives form and stature to the body, and which serves for the support of the soft parts and the attachment of muscles, is, with rare exceptions, all that is ever preserved of fossil animals. Because, therefore, students of extinct animals must rely so much, if not exclusively, upon the skeleton much attention has been given to the study of comparative osteology, the science of bones. Not only are most of the bones of the skeleton characteristic of the genus to which they belong, but the more general plan of the skeleton, or parts of the skeleton, is likewise characteristic of the larger groups. The paleontologist may become so expert in deciphering the characters of single bones, or even parts of bones—often all that are known of animals new to science—that he is able to hazard guesses as to the general structure of the skeleton to which they belong. But such guesses usually will approximate the real truth only in the degree that the bones upon which they are based approximate like bones of other animals that are better known. Not all parts of the skeleton are equally characteristic of the type of animal which possessed them. A tooth of a mammal may positively determine the species to which it belongs, while the toe bone of the same animal might not enable one to guess at its family, even. As a rule one can seldom be quite sure of the species of a reptile unless the larger part of the skeleton, or at least the skull, is available, although almost any bone of the skeleton, if one is expert, will permit a decision as to the family, if not the genus.

One must often depend upon the positions and relations of the bones, as found in the rocky matrix, for the final determination of many characters. One can, for instance, never be sure of the number of bones in the neck, trunk, tail, or feet of a reptile, until specimens have been found with all such bones in position. It is for this reason that much care is exercised in the collection of specimens of fossil animals, and especially of fossil reptiles, to preserve all parts of the skeleton, so far as possible, in the relations they occupied in the rocks until they can be studied in the laboratory. Many grievous errors have been made in the past by hasty inferences from fragmentary and poorly collected specimens.

Because of the reliance which must be placed upon the skeleton it will be necessary to speak somewhat in detail of its structure in the reptiles, and to use not a few terms in its description that are unfamiliar to the general reader. So far as possible technical terms will be avoided, though some must be used, as there are no equivalents in the English language for them. The reader may use this chapter as a sort of explanatory index or glossary for the better elucidation of the necessary details of the following chapters.

It is needless to say that the skeleton of a reptile is arranged on essentially the same plan as that of our own; the bones have the same names that they have in our own skeleton, but there are more of them, and the individual bones, as a general rule, are less highly specialized, that is, are not so well adapted for special functions. In a word, the skeleton of a reptile for the most part is generalized, though particular parts may be highly specialized for particular uses. As a rule, if not as a law, the course of evolution has been to reduce the number of parts and to adapt those which remain more closely to their special uses, either by increase in size, or by modifications of their shape and structure.

SKULL AND TEETH

The skull of reptiles is much more primitive or generalized in structure than is that of mammals, to such an extent, indeed, that there is yet much doubt as to the precise homologies of some of the bones composing it; and, inasmuch as the names were originally given, for the most part, to the bones of the human skull, there is still some confusion among students as to the proper names in all cases, a confusion that doubtless will not be wholly dissipated until we know much more about the early or more primitive reptiles than we do at present.

As in other parts of the skeleton, there has been a reduction in the number of parts of the reptile skull from that of the more primitive forms, and a better adaptation of those which remain for the special uses they subserve. This reduction in number has been caused in part by the actual loss of bones, in part by the fusion of contiguous ones. The most primitive reptiles had no less than seventy-two separate bones in the skull;[1] the human skull has but twenty-eight inclusive of the ear bones. There is but little variation, either in the number or in the relations of bones, in the mammalian skull. If one knows the human skull thoroughly he can easily understand the structure of the skull of any mammal. The same cannot be said of the skulls of reptiles; one would be greatly puzzled in the comparison of the skulls of turtles and crocodiles, if he knew nothing about other forms. And it is safe to formulate another general law in evolution here: Characters which have been longest inherited are least liable to change. The earliest reptiles had at least four pairs of bones which have disappeared in all later reptiles; and they had some bones in pairs which have fused in later reptiles, either with their mates or with contiguous bones. The crocodile has at least two pairs of bones which have disappeared in turtles. On the other hand, the turtle has at least one pair of free bones which have been fused with adjacent bones in the crocodiles, and one pair that is fused which is free in the latter. The lizard has one pair of bones that has been wholly wanting in other reptiles for millions of years, while on the other hand it has lost some bones that are present in all other modern reptiles. The four parts of the occipital bone of mammals, basioccipital, exoccipitals, and supraoccipital, are almost invariably free and there is a single occipital condyle, except in the Theriodontia.

In this reduction or fusion of parts, or in addition thereto, there has been a general lightening-up of the whole skull-structure in reptiles from the rather massive and protected form of the older to the lighter, less protected, and more fragile type of the later ones, since speed, greater agility, better sense organs, and doubtless greater brain power have rendered unnecessary or useless the older kinds, just as modern methods and modern arms have rendered useless the coat of mail of the Middle Ages.

The old reptiles had a continuous covering or roof for the skull, pierced only by the openings for the nostrils in front—the nares—the orbits for the eyes near the middle, and a smaller median opening back of them for the so-called “pineal eye.” The temporal region, that is, the region back of the orbits on each side, was completely roofed over by bone for the support and protection of the jaw muscles. In later reptiles this region has been lightened, either by holes that pierce it or by the emargination of its free borders, as in the turtles. The openings have occurred in different ways, and with the loss of different bones in various lines of descent. In one large group of reptiles, comprising the pterodactyls, dinosaurs, phytosaurs, crocodiles, and rhynchocephalians, there are two openings on each side, called the supratemporal and lateral temporal vacuities. In another still larger group there is a single vacuity on each side, all members of which it has been thought were markedly related to each other. Some of these, the lizards, snakes, and mosasaurs, the ichthyosaurs, and probably the proganosaurs, have the single opening high up on the side, corresponding apparently to the supra temporal vacuity of the double-arched forms, as those with two openings are called. Many others, however, like the whole order Therapsida and the Theromorpha, have the single opening lower down and bounded differently; their relationships are doubtful, since it is very much of a question how the single opening has arisen. There have been many theories to account for the origin of the temporal vacuities, but all are yet speculations. Notwithstanding these doubts, which more recent discoveries have intensified, there can be none that the structure of this region of the skull offers important and reliable characters for the classification of the reptiles into the larger groups, but, unfortunately, we are very uncertain yet as to what this classification should be. We are confident that all those reptiles having two temporal vacuities on each side are related to each other; we are yet very much in doubt as to the classification of all other reptiles, or at least all others having only a single temporal vacuity on each side.

Better evidences of relationships, or the absence of relationships, are offered by the presence of certain bones in the skulls in some orders that are lost in others, since it may be accepted as an axiom that new bones have not appeared in the skulls of reptiles, birds, or mammals; and that no bone which has once disappeared has ever been functionally regained by the descendants of those that lost it. The presence, then, of an extra bone in the temporal region of the lizards or the ichthyosaurs is proof that they have had a long and independent descent from reptiles which possessed it.

The mandible of the earliest reptiles was composed of not less than seven separate and distinct bones, as shown in the accompanying figures. The mandible of no modern reptile has more than six, and some have fewer. The mandible of mammals is composed of a single bone, the dentary; those reptiles, the Theriodontia, which doubtless were ancestral to the mammals in Triassic times, have all the bones, except the dentary, much reduced, or even vestigial. The prearticular bone, as shown, so far as known, has been absent in all reptiles since Triassic times, except the ichthyosaurs, plesiosaurs, Sphenodon, and turtles, all reptiles of ancient origin. The coronoid bone primitively extended the whole length of the teeth on the inner side; in all reptiles, except the plesiosaurs, since Triassic times it is either reduced to a small bone back of the teeth or is absent. So also the splenial has been greatly reduced in size in all later reptiles and may be wanting as in Sphenodon and modern turtles. The articular of reptiles, it is now generally believed, is represented in mammals by one of the ear bones, the quadrate by another.

The teeth of reptiles are of much less importance, as a rule, in the determination of relationships than are the teeth of mammals. Rarely are their shapes of specific, and often not of generic, importance, though their number and relative sizes may be. The teeth of mammals, as a rule, are forty-four or less in number, and they are always inserted in distinct sockets in the jaw bones. Among reptiles they are indefinite in number, and may be attached to any of the bones of the palate and sometimes also to the coronoid of the mandibles. Furthermore, except in those reptiles related to the immediate ancestors of the mammals, they are alike or nearly alike in the jaws, that is, homodont, not distinguishable into incisors, canines, and molars. They may be inserted in separate sockets (thecodont), in grooves, or simply be co-ossified to the surface of the bone (acrodont). And they are usually reproduced indefinitely by new teeth growing at the side of the base or below them. More usually they are pointed and curved; sometimes they are flattened, with sharp cutting edges in front and behind in the more strictly carnivorous reptiles; in those of herbivorous habits they are more dilated and roughened on the crown, not pointed; in not a few they are low, broad, and flat and are used only for crushing the hard shells of invertebrates. With the very few exceptions among certain dinosaurs, they never have more than one root for attachment. The evolutional tendency for reptiles, as for the mammals, is to loose teeth, especially those of the palate. Among living reptiles it is only the most primitive types, such as the lizards, snakes, and the tuatera, which have teeth on the palatal bones, and in none are there teeth on the vomers, as was the rule in the ancient reptiles. The lizards may have them on pterygoids and palatines, and the tuatera has them on the palatines only. There may be as many as eighty on each jaw, above and below, and hundreds of smaller ones on the palate, or they may be reduced in number to five or six, or even to a single one; some reptiles, like the turtles and later pterodactyls, have none. The teeth of reptiles are composed of the same kinds of tissues as are the teeth of mammals, that is, of dentine and enamel, but the enamel is always thin, perhaps because the teeth are so easily replaced that a thicker protective covering is not needed. The arrangement of the dentine in primitive reptiles is complicated, that is, plicated or folded in labyrinthine figures, like that of many stegocephalian amphibians, the Labyrinthodontia, especially. This labyrinthine structure of the dentine persisted longest in the ichthyosaurs.

VERTEBRAE AND RIBS

The spinal column or backbone of reptiles, as in all air-breathing vertebrates, is made up of a variable number of separate segments called vertebrae, permitting flexibility. Each vertebra is composed of a body, or centrum, and an arch on the dorsal side for the protection of the spinal cord. Various projections from the vertebra, called processes, serve for the attachment of ligaments or muscles, for articular union with adjacent vertebrae, or for the support of ribs, and these processes have characteristic differences in different reptiles. The pair in front and behind, for articulation with the adjoining vertebrae, may become obsolete or even lost in swimming reptiles, as we shall see; they are called zygapophyses. In not a few reptiles there is an additional pair for zygapophysial articulation in front and behind, called zygosphene and zygantrum, for the greater strengthening of the column; they are especially characteristic of snakes and certain lizards. In certain other reptiles, especially the long-necked dinosaurs, there is an additional pair arranged differently from the zygophene, that have received the names hyposphene and hypantrum.

On the top of the arch is the spine or spinous process, which may vary enormously in size and length; sometimes it is flattened or dilated above for the support of an exoskeleton, or it may be heavy and massive for the attachment of strong muscles and ligaments. In the modern basilisk lizards and in the ancient Dimetrodon and Edaphosaurus from the Permian rocks of Texas these spines are of enormous length, some of them nearly four feet long in reptiles not twice that length. Slender crawling reptiles usually have no spines, or only vestigial ones. On the sides of the arch there may be a distinct transverse process for the articulation of the rib.

In all early reptiles the ends of the body or centrum are concave, as they are in nearly all fishes. Such a conformation, called amphicoelous, gives great flexibility to the spinal column, but only moderate strength, since the intervening spaces are filled with cartilage in life. In all living reptiles, with few exceptions, the body is concave, like a saucer, in front and correspondingly convex behind, and the intervening cartilage has largely disappeared. Such a mode of union, called procoelous, adds greatly to the strength of the backbone, enabling it to receive greater shocks or greater pressure without dislocation; or to sustain the greater strain of muscles used in running swiftly or in climbing. Among living reptiles, only the gecko lizards and the tuatera have biconcave vertebrae. Some extinct reptiles, such as some of the dinosaurs, animals that walked erect upon their legs, had their vertebrae convex in front and concave behind (opisthocoelous). Birds, though walking erect, have a very different and more complicated articulation of the cervical vertebrae, and certain reptiles, like the turtles, have very complicated cervical vertebrae.

In the embryos of all vertebrate animals there appears first an elongated fibrous rod, called the notochord, in the place of the future spinal column. This rod may persist through life, never ossifying, as was the case with all the earliest fishes, and is the condition in some living ones. As the embryo grows, however, the separate segments, or vertebrae, ossify about this rod in all reptiles, forming bony rings, perforate at first in the middle for the more or less constricted notochord. This stage was the permanent condition in all the earliest reptiles and in some later ones. Such animals are said to have notochordal vertebrae, the notochord more or less continuous, like a string of beads, the beads representing the enlargements between the contiguous vertebrae.

In many early amphibians, and probably in all the earliest ones, as well as in the fishes from which they were derived, the vertebra is more complicated in that it is composed of at least three pairs of separate bones, two of which united with each other, the third finally disappearing in modern animals, or at the most represented by a mere vestige called the intercentrum. The dorsal pair of these bones, called the neurocentra, forms the arch of the vertebra. The ventral posterior pair, called the pleurocentra, increases in size and unites to form the centrum or body of the vertebra; while the ventral anterior pair, early united with each other, is called the hypocentrum or intercentrum, persistent in all early reptiles as a vestige between the centra on the ventral side. This divided condition of the vertebra is persistent in the first vertebra, the atlas of all higher animals, in which the so-called body is the hypocentrum or intercentrum, the arch is the neurocentrum, while the pleurocentra have fused more or less with the anterior part of the next vertebra, the axis, to form the so-called odontoid. That this is the real explanation of the structure of the atlas is proved by the various stages of its evolution in the reptiles, from the earliest (Fig. 15) in which it scarcely differs from rhachitomous—as this structure is called—vertebrae of an early amphibian, to the modern in which the structure is nearly like that of mammals.

In front of the atlas, that is, between it and the skull, there was, in all early reptiles, as well as in some later ones, like the crocodiles and tuatera, the remnant of what is believed to have been another vertebra, of which only the arch remains, and which is called the proatlas. In its earliest condition it articulated with the skull in front and the arch of the atlas behind.

As in mammals, the vertebrae of the different regions have received distinctive names, cervical, dorsal, lumbar, sacral, and caudal. The numbers of each region are far more variable than they are among mammals, the total number of vertebrae in the column varying from about thirty to more than five hundred, in certain snakes. Nor are the different regions always easily distinguishable, especially those in front of the sacrum. In the earliest reptiles there was practically no neck, and only two vertebrae, the atlas and axis, that properly can be called cervical. Very soon, however, the reptiles developed a longer neck with seven vertebrae, a number that has remained singularly constant in higher animals, especially in the mammals. In most modern reptiles there are from seven to nine; in a few lizards, five. But the number was much more inconstant among the older reptiles; some of the plesiosaurs had as many as seventy-six cervical vertebrae; some of the older lizards even had as many as eighteen.

Ordinarily the cervical vertebrae differ from those behind them only in the small size or fusion of their ribs; sometimes, however, as in the Protorosauria and Pterosauria, the vertebrae may be much elongated. The dorsal vertebrae of reptiles vary in number from ten in turtles and some dinosaurs to forty-three in Pleurosaurus; and under the name dorsal we include the so-called lumbar, as there is seldom any real distinction between the two series, save the smaller size or the co-ossification of the ribs of the latter.

The sacrum in reptiles primitively consisted of a single vertebra, which bore a large rib on each side for the support of the pelvis. Very early, however, a second or even a third vertebra was added to it from behind. The number two is the rule among reptiles, both ancient and modern; among crawling reptiles the number never exceeds three, but among ambulatory and flying reptiles the number may be as great as in any mammal.

The number of caudal vertebrae in reptiles is exceedingly variable, from a dozen or fifteen up to a hundred and fifty or more. In snakes but two regions are distinguishable, the caudal and precaudal, and the number altogether may reach nearly five hundred. With the exception of the first few basal caudal vertebrae (pygals) and the minute ones at the extreme tip, all caudal vertebrae of reptiles bear a slender, usually Y-shaped bone below in the interval between the centra, for the protection of the vessels and nerves. Because of their shape they have been called chevrons, and are really outgrowths from the intercentra.

The ribs of reptiles are of more importance in classification than one would suppose. The primitive rib was a slender, curved bone, with the vertebral end dilated to articulate continuously with the intercentral space—that between the centra and the anterior part of the arch. And this is the condition still remaining in the tuatera. Very soon, however, the lower end of the articular surface (capitulum) became separated from the upper (tubercle) by a notch, and the ribs became distinctly double-headed. And this mode of articulation is the rule among mammals. Among later reptiles, however, there were many modifications. In nearly all the head migrated a little backward on the centrum. By the loss of the tubercle in lizards, the head became truly single-headed, and attached solely to the body; and this condition is characteristic of the order Squamata. In another large group the head of the rib gradually migrated up on the arch and on the transverse process (diapophysis), so that both head and tubercle are attached to the diapophysis; and this condition is equally characteristic of the orders of reptiles known collectively as the Archosauria—the crocodiles, pterodactyls, dinosaurs, and phytosaurs. In the Sauropterygia, the ribs are single-headed and attached to the end of the diapophysis. Finally in most ichthyosaurs the capitulum and tubercle both articulate with the body of the vertebra.

Ribs primitively were probably attached to all the vertebrae to the end of the tail. In the earliest reptiles that we know they are present on all vertebrae as far back as the tenth or twelfth caudal only, those of the caudal for the most part co-ossified with the centra. The ribs of the neck vertebrae more quickly disappeared, or became fused with the vertebrae, and only in the crocodiles among living reptiles are there ribs on the atlas. The sacral ribs, on the other hand, became much larger and stouter and developed an articulation at their outer ends for the support of the ilium (Fig. 16).

The so-called ventral ribs are slender ossifications in the connective tissue under the skin, on the under side of the body, and are characteristic of most reptiles. The anterior ones doubtless fused together more or less to form the sternum or breast bone, which was otherwise absent in the early reptiles.

PECTORAL OR SHOULDER GIRDLE

Those bones which form the framework for the support of the anterior extremity in vertebrate animals are known collectively as the pectoral girdle. In our own skeleton there are but two on each side, or four in all, the scapula or shoulder-blade, and the clavicle or collar-bone. A third bone, however, is represented in all mammals by a mere vestige which early unites with the scapula and is called the coracoid process. In the lowest forms of mammals, the Monotremata, of which the Ornithorhynchus and Echidna are the only examples, not only is this coracoid bone largely developed, articulating with the sternum or breast bone, but there is an additional coracoid bone in front of this; and there is also an interclavicle. Indeed, the pectoral girdle in these mammals is more primitive or generalized in structure than it is in any living reptiles, composed of scapula, coracoid, metacoracoid, and clavicle on each side and an interclavicle in the middle. No living reptiles have the metacoracoid, and, as is the case with many mammals, some reptiles have no clavicles.

Primitively, that is, in all the old reptiles, the girdle is composed of scapula, coracoid, metacoracoid, clavicles, and interclavicle, while in some of the very oldest there is yet another bone, more or less of a vestige, derived from the ancestral amphibians and called the cleithrum or supraclavicle. The scapula is more or less elongated in crawling and climbing reptiles; more slender and bird-like in those which walked erect after the manner of birds and mammals; shorter and more fan-shaped in the swimming reptiles, as we shall see. In some pterodactyls, unlike all other known animals, the scapula articulated at its upper end with the backbone, giving a much firmer support for the anterior extremities. Only in those reptiles allied to the ancestors of the mammals has the scapula ever had a spine or projection on its dorsal side.

Of the two coracoid bones in the original pectoral girdle the posterior one began to disappear early and is entirely lost in all reptiles that lived later than Triassic times, though it still persists in the lowest mammals, as we have seen. In most later reptiles the remaining coracoid has become less firmly attached to the scapula than it was in the older reptiles. It usually has a small foramen piercing it near the middle of the upper border or end, the supracoracoid foramen. The clavicle, while more constant among reptiles than among mammals, has been lost in some, the Crocodilia, for instance, as also the dinosaurs and pterodactyls. The interclavicle is more constant in reptiles, a more or less T-shaped bone underlying the coracoids where they join, or the breast bone; but there were some reptiles that lost it, the dinosaurs and pterodactyls, for instance. In the turtles both the clavicles and the interclavicle form a part of the under shell or plastron.

The cleithrum is known in only a few of the old reptiles; it is a more or less slender bone which lies along the upper front margin of the scapula, articulating at its lower end with the upper end of the clavicle on each side.

The breast bone or sternum, while not properly a part of the pectoral girdle, may be mentioned here. In reptiles it is rarely well developed or even ossified, the flying reptiles known as the pterodactyls being the most notable exceptions. It was a comparatively late development in this class, the earliest ones not possessing it even in a cartilaginous condition. It was doubtless evolved from the more or less numerous and slender ossifications on the under side of the body called ventral or abdominal ribs, after the coracoids had become reduced and more slender. Whenever it is present the coracoid articulates with it on each side in front. In most lizards it remains as a cartilage throughout life.

ANTERIOR EXTREMITY

The upper arm bone, or humerus, like most other bones of the extremities, has been greatly modified by the habits of the different reptiles. In running and climbing reptiles it is always slender, while in burrowing reptiles it is short and stout and much expanded at the extremities, like the humerus of the mole among mammals. And we shall also see how greatly modified it was among the swimming reptiles. The humerus of flying reptiles has an enormous process on the side, corresponding to the attachment of the deltoid muscle. The head of the humerus, for articulation with the glenoid cavity of the scapula, is rounded in all reptiles, except the pterodactyls, and the articulation is always at the extremity. At the lower extremity the protuberance at the outer or radial side is called the ectocondyle; that on the inner or ulnar side, the entocondyle. Between the two at the end are the articular surfaces for the radius and ulna, the capitellum and trochlea. A little above each of these condyles there is usually, on one side or the other or on both, a foramen or hole for the passage of arteries or nerves. That on the inner side, which is characteristic of all early reptiles and of many mammals, is called the entepicondylar foramen; that on the outer side, the ectepicondylar foramen; the latter is present in the lizards, and both are found in the tuatera and some of the early reptiles.

The radius and ulna are always distinct bones in reptiles, and always freely movable on each other; they are usually shorter than the humerus, but in some springing and climbing reptiles they are quite as long.

The carpus or wrist of reptiles consists primitively of eleven distinct, irregularly shaped bones, which articulate more or less closely with each other in three rows. Those of the first row, all true carpals, are known usually as the radiale, intermedium, ulnare, and pisiform, corresponding quite with the bones of the human wrist known as the scaphoid, lunar, cuneiform, and pisiform. The second row has but two bones, on the radial side, known as the centralia; while the third row has a bone to correspond to each of the metacarpals, five in number, and collectively known as the carpalia. Some or indeed all of these bones may be either absent or unossified, that is, remaining through life as nodules of cartilage. Seldom, however, are there less than nine bones in the carpus of reptiles.

The metacarpals, like the digits, primitively were five in number, and seldom are there less, though the fifth is sometimes lost, and rarely also the first. They are more or less elongate bones, increasing in length from the first to the fourth, with the fifth usually shorter. The first and the fifth are usually more freely movable on the wrist than are the other three.

The number of joints or phalanges in the fingers of all primitive reptiles is that of the modern lizards and the tuatera, that is, two on the first finger or thumb, three on the second, four on the third, five on the fourth, and three on the fifth. The crocodiles have one less phalange on the fourth digit; the turtles have usually two less on the fourth and one less on the third, that is, with precisely the same arrangement that is found in our own fingers and that of mammals in general, two on the thumb and three on each of the other fingers. As exceptions the river turtles have four bones in the fourth digit. And this mammal-like and turtle-like arrangement of the phalanges was that of those early reptiles, the Theriodontia, from which the mammals arose. The last or ungual phalange of reptiles is usually claw-like, that is, sharp, curved, and pointed, but sometimes it is more nail-or hoof-like.

PELVIC OR HIP GIRDLE

The pelvic girdle or pelvis in reptiles and higher animals consists of three bones on each side, often closely fused in adult reptiles and together known as the innominate bone. The upper or dorsal one of these three bones—that to which the sacrum is attached—is the ilium; the one on the lower or ventral side in front is the pubis; and that on the ventral side behind is the ischium. On the outer side, where these three bones meet, is a cup-like depression, sometimes a hole, called the acetabulum, for the articulation of the head of the thigh bone, homologous with the glenoid articulation of the pectoral girdle, which, as we have seen, was originally formed by three bones, the scapula, coracoid, and metacoracoid, the two latter bones, like the pubis and ischium, meeting in the middle line below. In all the primitive and early reptiles the pubis and ischium form a continuous plate of bone without holes in it, except a small one just below the acetabulum in the pubis, called the obturator foramen, and corresponding to the supracoracoid foramen of the coracoid. One may almost always recognize these two bones by the presence of the foramen. This “plate-like” condition of the pelvis has been lost in all late and modern reptiles by the appearance of a larger or smaller vacuity between the pubis and ischium, either paired, when it corresponds quite with the so-called obturator opening of mammals, or singly in the middle. This old-fashioned character, like the old-fashioned type of pectoral girdle, disappeared entirely about the close of the Mesozoic period, the Choristodera, described in the following pages, being the last of the kind.

The ilium in reptiles usually has a more or less prolonged process or projection turned backward by the side of the anterior caudal vertebrae, but in those animals which walked erect on the hind legs, the dinosaurs and pterodactyls, as also some of the more erect-walking reptiles ancestral to the mammals, this process was directed forward, as in birds and mammals. The crocodilia, unlike all other known reptiles, have the pubes excluded from the acetabulum, and they do not meet in a median symphysis. This character alone will distinguish any crocodilian from all other reptiles. But there is some doubt as to the homology of the bones usually called pubes in the crocodiles. Some of the bipedal dinosaurs have the pubis forked, the anterior part directed downward and forward, and not meeting its mate in a symphysis, the posterior process long and slender, lying below the long ischium, as in birds. Indeed, when this peculiarity of the dinosaurian pubis was first discovered, it was thought to be an evidence of the immediate relationship of birds; its structure is now interpreted differently.

POSTERIOR EXTREMITY

The thigh bone or femur in reptiles, like the humerus, is variable in size and shape. Only in those reptiles that walked erect is the articulation of the head set off from the shaft of the bone by a distinct neck. In others the articulation is at the extreme top of the bone, since the thigh bones are habitually turned more or less directly outward from the acetabulum and the long axis of the body. The more or less pronounced rugosities at the upper end of the femur, for the attachment of muscles, called trochanters, are not easily distinguishable into the greater and lesser, as in mammals. Sometimes, as in the erect-walking dinosaurs, there is a more or less pronounced process on the shaft lower down, called the fourth trochanter, for the attachment of caudal muscles. On the back part of the shaft there is a ridge or line for the attachment of muscles, corresponding to the linea aspera of the mammalian femur. The projections at the lower end on the sides are called condyles.

The two bones of the leg, or shin, are usually shorter than the thigh bones, though in running and leaping animals they may be quite as long or even longer. That on the inner or big toe side is called the tibia, and articulates with the distal end of the femur, but chiefly with its inner condyle. It has a more or less well-developed crest in front above for the attachment of the extensor muscles directly, since there never is a patella in reptiles, and only rarely sesamoid bones of any kind. The fibula, at the little-toe side of the leg, is usually more slender than the tibia, though it may be larger in swimming reptiles and even in some running forms. It disappeared in some of the later pterodactyls. Its upper articulation has a more gliding and somewhat rotary motion on the outer condyle of the femur, turning the foot outward in extension of the leg.

The tarsus of reptiles differs from that of mammals, in that the chief movements of extension and flexion of the foot upon the leg occur within the tarsus rather than between the tarsus and leg bones. Primitively the tarsus of reptiles consisted of nine bones, two in the first row, two in the second, and five in the third, but in all modern reptiles the bones of the middle row and the fifth one in the third row have disappeared; in some lizards and turtles the two of the first row are fused. The two bones of the proximal row correspond quite to the astragalus and calcaneum, the astragalus articulating with both tibia and fibula proximally, the calcaneum with the latter only. The oldest known tarsus of any vertebrated animal, one from the Coal Measures of Ohio, has this structure, while in all the early amphibians there were three bones, the tibiale, intermedium, and fibulare. Some of the later swimming reptiles, like the ichthyosaurs and plesiosaurs, have apparently this amphibian structure, with three bones that are usually called tibiale, intermedium, and fibulare, but it is very doubtful indeed whether they are homologically the same. In the middle row two centralia are known in one or two very ancient reptiles, but for the most part there is only a single centrale, and even that is usually lost in later reptiles. The third row, like the third row of the carpus, had a distinct bone for each digit originally, but the fifth one was very soon lost and has never reappeared. The structure of the digits and number of bones are quite like those of the hands, except that the fifth toe has four bones instead of three, that is, the phalangeal formula was 2, 3, 4, 5, 4. As a rule in terrestrial reptiles, as in terrestrial mammals, the hind foot is more specialized than the front ones.

Most reptiles have an external covering or exoskeleton of horny plates or scales or bony scutes. Horny scales are of course not preservable as petrifactions, though in many instances their actual carbonized remains or their impressions have been detected. Such information comes only rarely, though doubtless in the course of time we shall obtain it for most extinct reptiles. In the mosasaurs, for instance, very perfect impressions showing the detailed structure of the scales have been frequently found. Similar impressions were long since observed by Lortet in Pleurosaurus, and in not a few dinosaurs impressions of most wonderful perfection have been found. It is only in the water reptiles, probably, that all external coverings tended to disappear.

Bony dermal plates or scutes are less common among reptiles, though by no means rare. The turtles, as is well known, are almost completely inclosed in such an exoskeleton, bones which have coalesced more or less to form a box or carapace within which the head and limbs may be withdrawn for protection. In the modern crocodilians also the body is more or less protected by small bone plates forming rows on the back and sometimes on the under side. The ancient phytosaurs had similar plates. Not a few of the dinosaurs were more or less covered with bony scutes and sometimes with large bony plates or spines. Some modern lizards have bony plates over the body instead of horny scales.