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.
Fig. 102.–Skeleton of Triceratops prorsus. × 1⁄70. (After Marsh.)
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.
Fig. 103.–1, Atlas and axis of Crocodilus. 2, Atlas and axis of Metriorhynchus, a Jurassic Crocodile, see p. [439]. 3, Analysis of the first two cervical vertebrae of a Crocodile. 4, Diagram of the fundamental composition of a Reptilian or other Amniotic, typically gastrocentrous vertebra. Az, Anterior zygapophysis; B.D, basidorsal; B.V, basiventral; C1, C2, first and second centra, formed by the interventralia; Cp1, Cp2, articular facets of the capitular portions of the first and second ribs; I.V, interventral; N1, N2, first and second neural arch, formed by the basidorsalia (B.D in 4); Od, odontoid process = first centrum; Pz, posterior zygapophysis; R1, R2, ribs; Sp, detached spinous process of the first neural arch; t1, t2, facets of the tubercular portions of the first and second ribs; 1, 2, intercentra = basiventralia; 2 (in 3), second basiventral "complex or intercentrum," continued upwards as a meniscus or intervertebral pad; I, II, III, position of the exit of the first, second, and third spinal nerves.
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.