Allosaurus, from the Upper Jurassic of North America, with only three toes. Ischia and pubes united into one symphysis. Anterior extremities very short. Sacrum consisting of four vertebrae. Total length of some of the larger species about twenty feet.

Ceratosaurus nasicornis, from the Upper Jurassic of Colorado, is about seventeen feet long. The generic and specific names refer to the nasal bones, which are raised into an unpaired longitudinal crest. This, by its rough surface, suggests that it was covered by a horny sheath, or carried a horn. The large skull, about two feet in length, is armed with strong, slightly curved, laterally compressed, sharp teeth, unequal in size. The pre-orbital foramen is large, bordered above by the prefrontals, which are raised into prominent knobs. The supratemporal foramina are extremely small, the lateral foramina very large. The quadrate slants backwards. The sacrum consists of five vertebrae. The caudal vertebrae carry long and slender chevron-bones. The pubes and ischia are long and slender, each forming a separate symphysis at their broadened ends. The three metatarsals are elongated and fused with each other. There seems to have been some dermal armour in the shape of osseous plates, which extended in one series from the occiput over the neck.

Coelurus gracilis, of the Upper Jurassic of Wyoming, and closely allied forms in the Wealden of England, are remarkable for the pneumaticity of the centra and processes of their vertebrae, the bony parts of which are restricted to thin, hollowed-out shells, so that the whole skeleton must have been very light. Computed length of these imperfectly preserved creatures about five feet.

Hallopus victor, of the Upper Jurassic of Colorado. Anterior extremities very short, with only four fingers; posterior limbs very long and slender, especially the tibia; the much elongated metatarsals are separate, the first absent, the fifth much reduced, so that the foot is tridactyle; the calcaneum projects like a heel. The ilium is attached to two sacral vertebrae only; the pubes are slender, forming a narrow symphysis, while that of the ischia is broad. Most of the bones of this creature, which probably progressed by hops, are hollow. Total length about three feet, the length of the hind-limbs being about nine inches.

Compsognathus longipes, of the Upper Jurassic of Bavaria, is one of the smallest of all the Dinosaurs. It is most remarkable on account of its almost bird-like feet. The fibula is much thinner and somewhat shorter than the tibia; the latter is closely attached to, although not fused with the proximal tarsal bones, while the distal tarsals are fused with the united and much elongated second, third, and fourth metatarsals; the fifth is reduced to a short bone near the intertarsal joint; while the first is represented by its distal portion only, which is stowed away on the hinder aspect of the middle of the second metatarsal, and carries two phalanges. The three middle toes consist of three, four, and four phalanges respectively. Whilst the whole hind-limb is typically avian, the pelvis is quite different; the pubic bones are simple, slender, and directed forwards, forming a symphysis with their whole distal halves, and broadening out distally into a horizontal process directed towards the symphysis, which is likewise formed by the fusion of the inner surfaces of the thin and rather flat ischia. The fore-limbs are only half the size of the hind-limbs. The neck consists of about ten vertebrae, mostly with long and pointed ribs. Tail long with well-developed chevrons. The skull is long and pointed, composed of thin bones, which have lost most of the sutures; with large lateral, temporal, and pre-orbital, but without supratemporal, foramina. Premaxillae, maxillae, and mandible with numerous slender and rather long, conical, alveolar teeth.

Order III. ORTHOPODA

The so-called pre-pubis is homologous with the pubis of most recent reptiles, and with the pectineal process of birds, while the "post-pubis" is homologous with the processus lateralis of Chelonians and Saurians, and with the "pubis" of birds. The right and left halves of the pubis remain widely asunder ventrally. In many cases the post-pubis, always directed obliquely backwards, lies closely against the shaft of the ischium, which always forms a distal syndesmosis, or a symphysis, with its fellow. The fore-limbs are usually very short, provided with five or four short and strong fingers. The hind-limbs are long and strong, mostly with three, sometimes with four functional short toes, either plantigrade (Stegosauri) or digitigrade (Ornithopoda). Femur with an inner distal, or fourth, trochanter. The dentition is of the herbivorous type, restricted to the dentaries of the mandible and to the maxillary bones, leaving the whole or the greater part of the premaxillaries free. The additional "predentary" piece of the mandible is possibly a calcified, but originally horny, pad. The teeth are greatly compressed laterally, and finely serrated, but are much ground down by use; several rows of successional teeth lie on the inner or lingual side. The skull is strongly built, with large anterior nasal openings; pre-orbital foramina very small or absent; orbits completely encircled by bones; supratemporal foramina small, lateral foramina large. Quadrate large, vertical or slanting slightly forwards. The vertebrae are solid, not hollow; sacrum consisting of four, five, or more vertebrae; ribs bifurcated, the capitula carried either by the centra, or moved up to the diapophyses of the neural arches; chevron-bones numerous, and frequently long, especially on the anterior half of the long and heavy tail.

Orthopoda occur from the Lias to the Upper Cretaceous, both in Europe and in North America. The name Orthopoda, invented by Cope in 1866, is appropriate for obvious reasons; it comprises the Stegosauri and Ornithopoda of Marsh (1881). The latter term is not very fortunately chosen, considering that the whole hind-limb of the Theropodous Compsognathus is far more ornithic than that of any three-toed Ornithopoda, in which the tarsalia rarely fuse with the tibia and never with the metatarsals. To apply the term Ornithopoda to the whole order is quite unjustifiable, unless it is meant to apply to the strikingly bird-like configuration of the pelvis.

Sub-Order 1. Stegosauri.–The fore- and hind-feet are plantigrade, or nearly so, the metapodials being but little elongated, with more than three functional digits. The bones of the limbs are solid. The ribs of the trunk are bifurcated, and are carried by the diapophyses of the neural arches. The body, especially the back, is protected by dermal bony plates, which are not connected with the internal skeleton.

Scelidosaurus harrisoni. One nearly complete skeleton, about 11 feet in length, from the Lias of Lyme Regis. About twenty-four pre-sacral vertebrae, of which six or seven belong to the neck, four sacral and about forty caudal vertebrae. Four fingers, four toes, with 2, 3, 4, 5 phalanges, the fifth metapodials being quite vestigial; the hallux and pollex are very short, so that the foot at least is functionally tridactyle. The tarsal bones remain separate. The head is very small. Two rows of ridged bony plates extend from the neck over the back, and converge into one row upon the long tail; smaller plates, arranged in many rows, seem to have protected the sides and under parts. Hylaeosaurus and Polacanthus of the English Wealden are allied forms.

Stegosaurus, with several species from the Upper Jurassic of Colorado and Wyoming, and others, e.g. S. armatus (= Omosaurus), from the Kimmeridge Clay of Wiltshire in England. The head is relatively very small, and the brain is surpassed several times in thickness by the huge sacral swelling of the spinal cord. Teeth numerous and small. All the cervical and trunk-vertebrae carry bifurcated ribs, those of the trunk being carried entirely by the very high neural arches. The fore-limbs are only about half the length of that of the hind-limbs, so that these creatures, which were undoubtedly quadrupedal, must have had a very peculiar gait, standing with the head, neck, and shoulders much lower than the arched back and pelvic region. The ulna has a strong olecranon; the hand has four functional fingers. The pre-acetabular portion of the ilium is much elongated; the pre-pubic branch stands horizontally, while the post-pubis is closely adpressed to the ischium. The astragalus is fused with the tibia, the calcaneum with the fibula. The foot has only three short toes, protected, like the fingers, by hoofs. The dorsal dermal armature consists of very high, crest-like plates. S. ungulatus of North America has a computed length of 28 feet, with the hind-limbs about 7 feet long. This creature was nearly 10 feet high, when measured from the ground to the tips of the dermal crests on the middle of the back. These bony, laterally compressed plates are themselves nearly 3 feet high, and are replaced, on the hinder portion of the tail, by several pairs of pointed spikes about 2 feet in length.

Sub-Order 2. Ornithopoda.–The hind-limbs are distinctly digitigrade, usually with only three functional toes, protected by claws. The long bones are hollow. Femur with a long fourth trochanter. Without dermal armour-plates.

Camptosaurus.–Several species, up to 10 feet in length, from the Upper Jurassic and the Wealden of North America and England. Five fingers, with 2, 3, 3, 3, 2 phalanges and four toes, with 2, 3, 4, 5 phalanges, but the hallux is much shortened and does not touch the hard ground; astragalus and calcaneum separate.

Laosaurus of Colorado is a smaller form, intermediate in structure between the former genus and Hypsilophodon foxi from the Wealden of the Isle of Wight. A small creature, less than 5 feet in length. Four fingers, with 2, 3, 4, 2 phalanges; fifth metacarpal vestigial. Four toes with 2, 3, 4, 5 phalanges and long claws. Astragalus and calcaneum separate. Post-pubis very slender. Each premaxillary with five pointed alveolar teeth, leaving a wide median diastema; maxillaries with eleven, dentaries with ten laterally compressed blade-like teeth.

Iguanodon from the Wealden of England, Belgium, and Germany. Apparently two species, I. mantelli, about 16 feet, I. bernissartensis nearly 30 feet long. The premaxilla is quite toothless; the teeth of the maxillae and mandibles stand in close series, implanted in alveolae; they are spatulate, laterally compressed, with finely serrated edges, and slightly curved, the lower outwards, the upper inwards, and bear a general resemblance to those of Iguana, hence the generic name. There is only one functional set of teeth, and these are much worn down by use, but in such a way that, owing to the different curvature of the opposed teeth, the worn-down crowns form cutting, and at the same time crushing, almost triturating surfaces, indicating that these animals lived upon herbs. The gait of these creatures was upright, as shown by their spoors; the long almost vertical ischia, which form a padded symphysis, only slightly raised above the ground, suggest that this symphysis was used as a true sitting support, the animal resting upon it, the hind-limbs and the long tail. The latter, to judge from the long chevrons and the high neural spinous processes, must have been furnished with strong muscles. The whole tail was undoubtedly used as a balance during the upright position. Many of the tendons of the dorsal spinal muscles on the back and upper half of the tail are ossified. The post-pubic branches are very slender, distally much reduced, and, except at the obturator-foramen, separated from the ischia; the pre-pubes are very strong and broad. The femur has a fourth trochanter, a feature which induced the unfortunate late Paul Albrecht to declare that Iguanodon was a reptilian Duck! The tarsal bones are separate. The metatarsals and toes are reduced to three, with 3, 4, 5 phalanges respectively, the first being a mere styliform vestige. The anterior limbs are likewise very powerful, but are much shorter; the hands are adapted for grasping, possibly for defence and offence, as indicated by the pollex, which, although short, is transformed into a formidable spur-like weapon, firmly fixed at a right angle to the other four fingers, the phalanges of which number 3, 3, 3, 4; the second and third fingers were protected by hoof-like nails, the fifth finger is feeble, and stands somewhat apart. The whole vertebral column consists of more than eighty vertebrae, of which ten are cervical, eighteen thoracic and lumbar, while five or six are fused into the sacrum. The cervical vertebrae are opisthocoelous, and carry short ribs, except the atlas, which possesses two separate supra-dorsal pieces, which fill the gap between it and the occiput.

Many specimens of I. bernissartensis, which is now completely known, including even the hyoid bones, were discovered in 1878, in the Belgian colliery of Bernissart, between Mons and Tournai, close to the French frontier. The bones were in a fault or crack, filled with clay of Wealden age, about one thousand feet below the present sea-level, and there about thirty Iguanodons, all apparently adult, had become embedded. Five of them are now mounted in one of the public galleries of the Brussels Museum, of which these perfect monsters form one of the chief attractions. Having proved to be such a valuable find, they were claimed by the Government, on the ground that Iguanodons were not included in the license of the Coal Mining Company. The fact that not only I. bernissartensis, but also a few specimens of I. mantelli, already known from England, where the large form likewise occurs, were found in the same place, makes the specific differences somewhat doubtful; they are perhaps sexual.

Claosaurus of the uppermost Cretaceous strata of Wyoming, is one of the latest of Dinosaurs. It is nearly allied to Iguanodon, but has only three functional fingers, the fifth being absent, whilst the pollex is very short.

Hadrosaurus s. Diclonius of the same level as the preceding genus in North America, apparently also in the Middle and Upper Chalk of England and Belgium, has a most peculiar spoon-shaped bill, the premaxilla and the predental bone being spatulate and quite toothless. The teeth in the upper and lower jaws are numerous and small, and whilst one set of teeth is being ground down, the several successional series are already functional. H. mirabilis has in all about 2000 teeth; the total length of the skeleton is 38 feet, of which nearly 4 feet are taken up by the skull; in other respects this genus is allied to Iguanodon.

Ornithomimus, of the Upper Cretaceous of Colorado, is known only from its fore- and hind-limbs. The fore-limbs are short, with three fingers. The hind-limbs are very long and strikingly bird-like. The metatarsals, of which only the second, third, and fourth are developed, are much elongated; the proximal half of the third is pushed back between the second and fourth, and imperfectly fused with them, exactly as in young birds. The astragalus has a long ascending process, and is fused with the tibia. The fibula is very slender, distally much reduced; the calcaneum is represented by a tiny nodule; the terminal phalanges end in pointed claws. O. grandis must have reached a considerable size, to judge from its middle metatarsal, which is 60 cm. or 2 feet long. Until more is known of these extraordinary creatures, nothing definite can be said about their affinities. They may perhaps belong to the Theropoda.

Order IV. CERATOPSIA.

The teeth of the upper and lower jaws are alveolar, and have two roots. The fore-limbs are little shorter than the hind-limbs; pentadactyle and plantigrade, with broad hoofs. Femur without a fourth trochanter. Limb-bones solid. The skull is large, and remarkable for a pair of long frontal bony cores, which probably carried large, pointed horns; the parietal bones form a huge, horizontally broadened out crest, which extends backwards over the neck. Upon this cranial neck-shield follow small dermal bony plates. These miraculous creatures flourished during the Cretaceous epoch in Europe and in North America. Some, for instance, the American Triceratops flabellatus, reached a huge size, its skull alone measuring more than 5 feet in length, while that of T. prorsus is, including the neck-shield, about 7 feet long. The total length of this monster, the back of which stands about 8 feet high, is more than 20 feet. Other genera seem to have a well-developed dermal armour, e.g. Nodosaurus of the Middle Cretaceous period of Wyoming.

The Ceratopsia combine characters of the Sauropoda and of the Stegosaurian Orthopoda; in their pelvis they agree with the former, in the development of dermal armour and a predental bone they agree with the latter, while they differ from either by the possession of a rostral element.

Sub-Class VI.–CROCODILIA.

If we had to deal only with the recent Crocodilia the following would be an all sufficient diagnosis:–Four footed, long-tailed reptiles, with fixed quadrate bones, with teeth separately implanted in alveolae and restricted to the upper and lower jaws.

To define Crocodilia in general and to distinguish them from various extinct groups we have to resort to additional characters. The vertebrae are solid; the ribs of the neck and thorax possess a distinct capitulum and tuberculum; there is a series of loose, compound abdominal ribs; the humerus is devoid of an entepicondylar foramen; the iliac bones are broadened out and attached to two sacral vertebrae; the pubic bones are simple, not bifurcated, and neither they nor the ischia are ventrally united. The skull always has a strong, bony, quadrato-jugal arch. The possession of a longitudinal cloacal opening and of an anterior or ventral single copulatory organ can of course be asserted of recent forms only.

In spite of these many characters common to all Crocodilia, it is very difficult to separate the latter from the Dinosauria, the only absolute difference lying in the ventral pelvic bones. It is therefore most suggestive that the fore-limbs of the Mesozoic Crocodilia are so much shorter and weaker than their hind-limbs, a discrepancy which is not lessened before the Tertiary epoch. The Mesozoic Crocodilia were almost entirely marine; the strongly-developed ankle-joint (indicated already by such early forms as Aetosaurus and Mystriosaurus) must have been inherited from some terrestrial group with digitigrade tendencies and shortened hind-limbs. All this points to some Theropodous Dinosaurian stock of which the Crocodilia may well form an aquatic, further-developed branch. Loss of the pubic and ischiadic ventral symphysis is not a serious modification. So far as modern reptiles are concerned only the Chelonia and Sphenodon are related to the Crocodilia, whilst Monitors and other lizards resemble them only superficially. We divide them into three Orders.

Order I. PSEUDOSUCHIA.

The few members of this peculiar group of reptiles are all restricted to the Keuper or variegated marls, although they seem to have had a wide distribution, some having been found in Germany, others in New Mexico. They perhaps form an early side-branch of the generalised Crocodilian stock, which died out with the Jurassic age.

The skull is distinctly short and pointed. The premaxillaries are very small and are dorsally separated from each other by the large nasals, which also keep the maxillae widely asunder. The nostrils are latero-terminal, bordered chiefly by the nasals, below by the premaxillae and part of the maxillae. The orbit is bordered below by the strong jugals, in front by the prefrontal, above by a supra-orbital and a small postfrontal, behind by a postorbital, which, firmly connected with the jugal and squamosal, shuts off a supratemporal foramen. There is also a lateral temporal fossa, and a large hole enclosed by the lacrymal and the maxillary bones. The teeth are restricted to the anterior half of the jaws. The neck, back, and tail are covered by two rows of large and broad, closely-jointed bony plates; smaller plates protect the sides and the ventral surface. The vertebrae are still unknown.

Aëtosaurus ferratus of the Upper Keuper near Stuttgart is the best known. One of the greatest treasures of the Stuttgart Museum is a slab of sandstone, about 2 square yards in size, upon which lie huddled together twenty-four individuals of various sizes, the largest measuring 86 cm. or 2 feet 10 inches. They are in a beautiful state of preservation, and many of them are in the most life-like attitudes, just as if a mass of sand had fallen upon them and crushed them down, and as if they were struggling to get out.

Erpetosuchus and Ornithosuchus of the Elgin sandstone seem to be allied forms.

Order II. PARASUCHIA.

As the name implies, a collateral branch of the true Crocodilia. They are, like the Pseudosuchia, restricted to the Keuper formation. The vertebrae are mostly biconcave, sometimes with nearly plain, scarcely concave, central joints. The premaxillae are very long and powerful. The nostrils lie far back, rather near the orbits, on the top of the snout, within the anterior half of each nasal and almost above the choanae. The latter are situated in front of the palatine bones and are divided by a backwardly directed process of the vomer, which is plainly visible on the roof of the mouth. The palatines and pterygoids leave a wide median space between them. The pterygoids are narrow and have three processes, the antero-lateral of which joins the palatines and the maxillary bones (there being no separate ectopterygoid), the inner joins the basi-occipital, and the postero-lateral the quadrate.

The orbit is surrounded by the frontal, prefrontal, lacrymal, postorbital and postfrontal, while the strong jugal is excluded. The temporal region shows a lateral and a dorsal foramen; the latter opens backwards and above the occiput, being bordered in front by the parietal, laterally by the squamoso-occipital bridge.

The vertebrae are amphicoelous. The first and second vertebrae are devoid of ribs; the cervicals and first thoracics carry separate capitular and tubercular processes for the attachment of the ribs, while the ribs of the rest of the trunk are carried entirely by the long diapophyses, as in the modern Crocodiles. The dermal armour consists of two rows of broad, dorsal, and several rows of smaller, lateral, bony plates.

Belodon is by far the best-known genus, with several species in South Germany and North America, some of which reached a length of 10 feet, without ventral armour. The closely allied Stagonolepis of the Elgin sandstone in Scotland had dorsal and ventral armour. Other genera in the Triassic formations of India and North America.

Order III. EUSUCHIA.

Crocodilia in the stricter sense. The premaxillae are short and always enclose the nostrils. The choanae lie behind the palatines, in recent forms even within the pterygoids. They occur from the Liassic or Lower Jurassic period to the present time.

The direct ancestors of the Eusuchia are still unknown. They cannot have been developed from the Pseudosuchia, nor do we know intermediate stages which connect them with the Parasuchia. The nostrils, situated within the premaxillaries, always lie in front of the nasals, although these sometimes extend forwards and form a bony internasal septum fusing with the usual cartilaginous septum. The choanae, instead of opening immediately behind the vomer, are carried far back, owing to the formation of a secondary bony palate. In the Jurassic Crocodiles this roof is formed by the meeting of the palatine bones in the medio-ventral line, and the choanae open immediately behind. From Cretaceous times onwards this roofing is continued by the pterygoids, which likewise form a median suture; and the united choanae (which may, or may not, be divided by a thin bony septum) are pushed towards the posterior end of the pterygoids. Since the Jurassic times there exists also a tendency to enclose the Eustachian passages (the remnants of the first gill-clefts) by bone. In the earlier members they were still wide slits or open grooves on the ventral side of the basi-occipital bone. Since the Cretaceous epoch they have been transformed into bony canals and open through one median hole, situated between the basi-occipital and the basisphenoid, immediately behind the posterior symphysis of the dorsal portion of the pterygoids, which latter almost completely cover the basisphenoid. The vomer is not visible (except in Caiman niger), being covered by the ventral junction of the palatines and maxillaries. The broad, lateral wings of the pterygoids are connected by separate bones, the ectopterygoids = transpalatines = transverse bones, with the maxillaries, and in recent forms also with the jugals. Thus an extensive, very firm bony palate is produced; and the large palatal foramina, between the palatines, maxillaries, ectopterygoids and pterygoids, are closed by the same dense mucous membrane which cover the whole roof of the mouth.

The opisthotic and epi-otic bones fuse early with the lateral and with the supra-occipital bones; only the pro-otic remains longer as a separate element, perforated anteriorly by a large hole for the exit of the third branch of the trigeminal nerve. The basisphenoid is scarcely visible, being covered by the pterygoids. The presphenoid is large, continued forwards and upwards into the usually cartilaginous interorbital septum. Near the anterior and upper margin of the presphenoid is a large notch on either side for the passage of the optic nerve, the three eye-muscle nerves and the first branch of the trigeminal nerve. There are no separate orbito-sphenoids, their place being taken by membrane or cartilage in continuation with the interorbital septum, but the alisphenoids are large, abutting upwards against the frontals. Each prefrontal sends down a vertical process which joins the palatine of its side.

The configuration of the snout varies much. There are two parallel lines of development since the Jurassic epoch, namely, long-snouted creatures, of which two still survive as Gavialis and Tomistoma, and more broad and short-snouted members like the rest of the Crocodiles and Alligators. In opposition to the Parasuchia the elongation of the snout is effected by the maxillaries. The length of the nasals varies much, mostly in conformity with that of the maxillaries. As a rule they reach the premaxillaries but not always the nasal groove. In Gavialis they are short, far separated from the premaxillaries by the maxillaries, which meet in the dorso-median line. The orbit is bordered by the frontals, which at an early age fuse into an unpaired piece, and by the prefrontal, lacrymal, jugal, and postfrontal. At a deeper level the orbit is partly divided from the lateral temporal fossa by a strong column which is formed by the meeting of a downward process of the postfrontal with an inner process of the jugal, and an ascending process of the ectopterygoid (cf. Fig. 108, p. 458). This arrangement adds considerably to the strength of the skull. The lateral temporal fossa is bordered in front by the column just described; below by the jugal and the quadrato-jugal, which is firmly wedged in between the jugal and quadrate; behind by the quadrate; above by the postfrontal, which forms a strong superficial bridge with the squamosal. This rests upon and often fuses with the quadrate and an intervening transverse wing-like extension of the lateral occipital bone. By this squamoso-postfrontal bridge part of the original temporal fossa is divided into the lateral one just described, and a dorsal fossa. The latter is bordered by the postfrontal, squamosal, and united parietals. This dorsal temporal fossa is consequently not homologous with that of the Parasuchia, a vestige of which is however present in many, especially in young skulls of Crocodiles, in the shape of a narrow passage which extends backwards from the dorsal fossa, bridged over by the junction of the parietal with the squamosal, and bordered below by the occipitals.

The size of the upper temporal fossae stands in an inverse ratio to that of the lateral fossae. In the older Eusuchia the upper were the larger of the two. The temporo-mandibular muscle which lifts or shuts the lower jaw arises from the walls of the upper fossa, passes beneath the jugal arch, and is inserted into the supra-angular portion of the lower jaw. In the more recent Crocodiles this muscle is more and more superseded by the pterygo-mandibular muscle, which, arising chiefly from the dorsal surface of the much broadened-out pterygoid bone, fills the widened space between the latter and the quadrate, and is inserted into the outer surface of the os angulare of the lower jaw. This muscle, owing to its general disposition, is capable of much more powerful development and leverage than the temporo-maxillary muscle, which latter, being more reduced, allows the dorsal fossae to be more and more closed up by the surrounding bones.

The fossae are still comparatively large in the long-snouted genera Gavialis and Tomistoma, which live entirely upon fish and scarcely chew their food, whilst these holes almost completely disappear in some of the Alligators, namely in the broad- and short-snouted members, which, having a varied diet, taken from every available group of the animal kingdom, chew their prey.

The quadrate extends obliquely backwards, and is immovably wedged in and partly fused with the quadrato-jugal, the squamosal, and the lateral occipital wings. Between the latter and the quadrate remains a slit-like canal, well visible from behind, through which passes the continuation into the mandible of the columellar or ossicular chain of the auditory apparatus. Intricate passages, used as additional enlargements of the space of the middle ear, pervade the proximal portions of the quadrate and the roof of the cranium beneath the parietal bridges mentioned above, the two sides communicating with each other. The supra-occipital bone is visible from behind; its top is covered and partly fused with a continuation of the parietals, which are, like the frontals, fused into an unpaired mass. The occipital condyle is formed entirely by the basi-occipital bone, so far as the articulating facet is concerned, but it is supported on either side by a lamella from the lateral occipitals.

The two halves of the lower jaw form a symphysis of very variable length. Each half is composed of six bones. (1) The articulare, perforated in its upper, posterior, inner corner by a canal for the reception of the siphonium, a narrow tube of connective tissue, which connects the cavities of the middle ear with the large empty space enclosed within the lower jaw; (2) the angulare; (3) the dentary, which alone carries the teeth; (4) the splenial, a long splint-like bone on the surface of the inner or median side of the jaw, of variable length; (5) the operculare, the counterpart of the splenial on the outer side; (6) the supra-angulare, which forms the dorsal border of the lower jaw between the dentary and the angulare.

The teeth, which are more or less conical or compressed laterally, are deeply implanted in separate sockets. They are often shed throughout life, the successors lying on the median side, and with their caps partly fitting into the wide, open roots of the teeth to be expelled. The number of teeth in the premaxilla is universally five on either side in recent forms, but in a few species, e.g. Crocodilus niloticus and C. porosus, the second pair is lost with maturity and is not replaced. In the broad-snouted kinds, especially in the Alligators, most of the upper teeth overlap laterally those of the lower jaw. In most species of Crocodilus the overlapping is less marked and the teeth partly interlock, but the fourth mandibular tooth, generally the strongest and longest, is received into a lateral notch at the junction of the premaxillary and maxillary. Frequently those of the longer lower teeth which fit into pits of the upper jaw, gradually transform the pits into holes by continued pressure upon the bone, and in old specimens the tip of the lower tooth may even perforate and stand out above the skin of the snout.

The vertebrae are solid, but remnants of the notochord persist for a long time in the middle of the centra. These are still amphicoelous in the Jurassic Eusuchia, and there were probably considerable intervertebral portions of the notochord. From the Lower Chalk onwards the vertebrae are procoelous, with the exception of the first caudal vertebra, which has a knob at either end, so that naturally the posterior of the two sacral vertebrae is opisthocoelous. This peculiar formation of the first caudal is probably correlated with the flexibility of the tail.

Cartilaginous intercentral rings, pads or menisci, occur regularly throughout the vertebral column, unless they are abolished by fusion of adjoining vertebrae. It is most instructive to follow the attachment of the ribs in one and the same individual. The position of the capitulum, vertically below the tuberculum in the neck, changes in the thorax into one in which the capitulum lies anterior to the tuberculum and in the same horizontal plane with it. Moreover, whilst on the cervical vertebrae the capitulum is carried by the centrum (enclosing with the tuberculum a typical transverse canal for the vertebral artery, etc.), further back it moves its point of attachment upwards, lying right upon the neuro-central suture on the tenth and eleventh vertebrae. From the twelfth vertebra backwards both capitulum and tuberculum are carried by the transverse process or diapophysis of the neural arch. The ribs of the five or six lumbar vertebrae are merely vestigial or absent. The ribs of the two sacral vertebrae are very stout, fusing in the adult with both centrum and neural arch. Some of the anterior caudal vertebrae also carry ribs, attached across the neuro-central suture; long before maturity they fuse with their vertebrae, and then look like transverse processes. Most of the caudal vertebrae carry also a pair of chevron-bones, and these are continuous with the intercentral rings of cartilage.

The atlas and the epistropheus or axis are of supreme interest. Crocodiles are, in fact, the only animals in which these two vertebrae retain all their constituent hard parts in an almost undisturbed primitive condition (Fig. 103, 1-4). The basal piece of the atlas-ring, the first basiventral or intercentrum, carries a pair of long ribs attached by their capitular portions. A small knob near the dorsal edge of the rib occurs in many specimens, and is the last remnant of the tubercular portion. The latter was still complete in Jurassic Crocodiles, for instance in Metriorhynchus (Fig. 103, 2, t₁). The first centrum joins that of the second vertebra as its so-called odontoid process, not directly, however, but by the intercalation of the complete second basiventral, represented by a cartilaginous disc, and by a large unpaired pyramidal piece (Fig. 103, 3, ²). This, serially homologous with the ventral half of the atlas-ring, is the second basiventral intercentrum, wedged in from below between the odontoid process and the second centrum, with which it soon fuses. Moreover, it carries the capitulum of the second rib (2, Cp²), the tuberculum of which is articulated with a facet of the second neural arch in Jurassic Eusuchia (t₂). In recent Crocodiles this tubercular portion is much reduced, and, curiously enough, is attached to a knob which belongs to the odontoid piece or first centrum. This shifting explains the apparently anomalous condition that "the atlas of the Crocodiles carries two pairs of ribs, the second vertebra none." To complete the account of the atlas we have to mention the separate unpaired piece which lies upon the two neural arches. It is the detached neural spine, and not the remnant of a "pro-atlas."

The first and second ribs (R₁ and R₂), at least in the recent forms, are very long and are quite movable. Those of the next five cervical vertebrae are firmly fixed, short, and adze-shaped. The eighth and ninth are again long, and make the transition to the thoracic ribs, which are mostly eight in number, some with uncinate processes. Then follow several shorter or floating ribs, mostly two or three pairs. The next following three presacral vertebrae carry no ribs. The two sacral and the caudal ribs have already been mentioned.

As a rule the vertebral column of recent Crocodiles, Alligators, and Gavials is composed of twenty-six precaudal vertebrae (namely, nine cervical, fifteen thoracic and lumbar, two sacral), and about thirty-four to forty or more caudal vertebrae. Individual variations, including lop-sided attachment of the iliac bones, are by no means uncommon.

The sternum remains cartilaginous. It consists of an anterior rhomboid portion, which carries the coracoids and two pairs of ribs, and a posterior longer and narrower portion formed by the median fusion of the next following five or six ribs. Posteriorly the sternum bifurcates, each half carrying two or three ribs, of which the last sometimes loses its proximal connexion, and thus appears as a xiphisternal process. Ventrally, upon the anterior part of the sternum lies the longitudinal, originally paired, episternum. The shoulder-girdle consists of the coracoids and the scapulae, which fuse with each other into one bony piece on each side. A pre-coracoid is indicated in fossil forms by a notch in the coracoid.

The space between the posterior end of the sternum and the pubic bones is occupied by the so-called abdominal sternum, composed of seven pairs of ossifications, resting upon the ventral side of the rectus abdominis muscle. Each pair consists of two closely apposed pieces, while the right and left remain separate in the median line. The last pair is much stronger than the rest, is more deeply imbedded in the rectus muscle, and is loosely connected with the anterior margin of the two "pubic" bones.

The limbs are built upon the typical terrestrial pentadactyle type, but were in the Jurassic species undoubtedly more adapted to swimming locomotion. The fore-limbs were conspicuously shorter and smaller than the hind-limbs, and it is only since Tertiary times that the difference has decreased to a great extent. Ulna and radius remain separate. The proximal row of carpal bones consists now of the ulnare and radiale, both strong and distinctly elongated. On the outer side, between ulna and ulnare, lies a pisiform bone. Upon the radiale follows a compound bone, often imperfectly ossified towards the median side, and consisting of the first distal carpal, the centrale, and the intermedium. The third, fourth, and fifth carpals are fused into one mass. The second distal carpal remains separate. All five fingers are present and well developed. The number of phalanges of the pollex is two, of the others three, four, four and three respectively. During the embryonic development the number of phalanges of the fourth and fifth finger increases temporarily, to as many as seven on the fourth, to five or six on the fifth finger. Before the young animal is hatched the numbers are reduced again, chiefly by fusion of adjoining phalanges. This hyperphalangeal condition, typical of Plesiosauri, Ichthyosauri, Cetacea, and several other absolutely aquatic animals, naturally suggests the descent of the present Crocodiles from more essentially aquatic ancestors, but hitherto no trace of supernumerary phalanges has been found in any Jurassic Eusuchia, nor in the Parasuchia and Pseudosuchia.

The composition of the pelvis is difficult to understand. It consists in the adult stage of three separate bones, of which two only partake in the formation of the acetabulum. The broad ilium sends out two processes; the posterior and stronger articulates with the ischium, which sends out a short and stout process towards the anterior process of the ilium, enclosing a foramen. This process contains a separate centre of ossification, possibly homologous with the true pubis, while each club-shaped bone, loosely attached to it and directed forwards, generally called the pubis of the Crocodiles, would then be equivalent to an epipubis. Neither the "pubes" nor the ischia form a ventral median symphysis.

The femur is devoid of a prominent inner trochanter. Tibia and fibula are of almost equal strength. The tarsal elements are, in the adult, reduced by fusion to five bones. The fibulare is transformed into a typically projecting, heel-shaped calcaneum, while the intermedium is fused with the tibiale into a broad astragalus. The first, second, and third distal tarsalia are much reduced towards the inner side, and form one wedge-shaped, partly cartilaginous mass. The fourth tarsale lies between the fibulare and the fourth metatarsal, while the fifth tarsale is hook-shaped and loosely attached to the outer side of the fourth. It has lost its metatarsal and the rest of the fifth finger. Embryos are hyperphalangeal, the fourth toe developing six phalanges, and there are traces of the fifth toe. The numbers are ultimately reduced to 2, 3, 4, 4, 0 on the five toes. The fourth toe remains without a claw.

Skin.–The epidermal horny layer is not shed periodically nor in pieces; the wear and tear is made good imperceptibly. The scales, which cover the whole body, have a hard, horny, waterproof covering, but between them the skin is soft. Each scale of the sides, belly, and tail, and especially those of the lower jaw, shows a little dot or pit. At this spot the epidermis is not cornified or thickened, and a nerve with sensory corpuscles ends beneath the bottom of the pit. Sometimes these pits are filled with débris of cells, and on the lower jaw, especially on the chin, these organs, instead of forming pits, are raised into little wartlike prominences.

The scutes or dermal portions of the scales consist of thickened, cutaneous connective tissue, and are more or less extensively ossified, thus forming a proper dermal armour. In most recent Crocodilia the armour is restricted to the back, with occasional osseous plates on the throat, as in Osteolaemus; regular although thin ossifications in the ventral scutes occur in the Caimans only. The Crocodile and Alligator skins of commerce consist entirely of the tanned cutis, minus the epidermis and the horny coverings of the scutes. In some fossil genera the ventral armour was extensively developed, especially in Teleosaurus, in some genera to the exclusion of dorsal ossifications. The armour of the recent forms consists, so far as the large scutes are concerned, of a considerable number of scutes, which are arranged in transverse rows, each row corresponding with one skeletal segment of the trunk proper. Mostly there is a detached cluster of scutes on the back of the neck. On the trunk some of the scutes are larger and more crested than others, and form in their totality a variable number of longitudinal rows. The median pair is generally the most conspicuous on the back. Some of the more lateral rows of keeled scutes converge more and more towards the tail, the inner rows drop out imperceptibly, and two lateral rows combine on the middle of the tail into an unpaired series of vertical blades. These are no longer bony, but show more strongly developed horny sheaths; they are very flexible, and transform the tail into an effective propelling organ.

Most of the larger scutes and the upper surface of the bones of the skull have a peculiar gnawed-out, almost honeycombed appearance, as is usual wherever most of the cutis itself is transformed into bone or co-ossifies with underlying bone, while the uppermost layers and the horny layer of the epidermis are much reduced and thinned out.