In many reptiles large pre-orbital vacuities occur; they are specially large in the Pterosauria and in some of the Crocodilia and Dinosauria (fig. 35, 3). In some Pterosauria they are confluent with the orbits.
The premaxillae are usually separate, but sometimes, as in some Ophidia (fig. 51, 1), Chelonia, Lacertilia (Agamidae), and Dinosaurs (Ceratopsia) they are united. In the Dinosaur Hadrosaurus they are exceedingly large and spatulate. In the Rhynchocephalian Hyperodapedon they are drawn out into a strongly curved beak.
As regards the mandible, sometimes, as in most Rhynchocephalia, Ophidia and Pythonomorpha, the rami have only a ligamental union; sometimes, as in Crocodilia, the Rhynchosauridae and the majority of Lacertilia, they are suturally united. In Chelonia (fig. 28, B, 12), and apparently in Pterosauria, the two dentaries are completely fused together. The sutures between the various bones of the lower jaw usually persist, but in Ophidia those between the angular, supra-angular, articular and coronoid are obliterated. There are sometimes large vacuities in the mandible, as in Theromorpha, Crocodilia, and some Dinosauria. In Iguanodon, Polyonax, Hypsilophodon and Hadrosaurus among Dinosaurs the mandible has a predentary or mento-meckelian bone which, in some cases at any rate, was probably sheathed in a horny beak.
The principal part of the auditory ossicular chain is formed by a rod-like columella. The development of the hyoid apparatus varies, and it often happens that the first branchial arch is better developed than is the hyoid arch. In the Crocodilia and Chelonia there is a large basilingual plate or body of the hyoid (fig. 53, 1); but while in the Crocodilia the first branchial forms the only well-developed arch, in the Chelonia the first and second branchials are both strongly developed, and the hyoid is often fairly large.
The Ribs.
Fig. 53. Hyoids of an Alligator (Caiman latirostris) (to the left) and of a Green Turtle (Chelone midas) (to the right) × 5/8. (Brit. Mus.)
The cartilaginous portions are dotted.
| 1. basilingual plate or body of | 3. first branchial arch (anterior |
| the hyoid. | cornu). |
| 2. hyoid arch. | 4. second branchial arch (posterior |
| cornu). |
Ribs are always present, and may be attached to any of the precaudal vertebrae. In most reptiles the posterior cervical vertebrae bear ribs, while the atlas and axis are ribless; in Crocodiles and Geckos, however, ribs are borne even by the atlas and axis. On the other hand, in the Chelonia none of the cervical vertebrae bear obvious ribs. In the following groups the thoracic ribs have both capitula and tubercula—Theromorpha, Ichthyosauria, Crocodilia, Dinosauria, Pterosauria. In the other groups each rib articulates by a single head, and the position of the facet is subject to a considerable amount of variation, thus in the Squamata it lies on the centrum, and in the Sauropterygia on the neural arch, while in the Chelonia the rib articulates with the contiguous parts of two centra instead of directly with one.
In most reptiles a greater or smaller number of ribs are united ventrally with a sternum; but in snakes a continuous series of similar ribs, all articulating freely with the vertebral column, extends from the third cervical vertebra to the end of the trunk. The number of ribs connected with the sternum varies from three or four in Lizards to eight or nine in Crocodiles. Those which reach the sternum are nearly always divided into vertebral, sternal, and intermediate portions, and as a rule only the vertebral portion is completely ossified. In Crocodiles a number of sternal ribs are connected with a cartilaginous arch, which is attached to the hind end of the sternum, and represents the xiphisternum. The sacral ribs connecting the vertebral column with the ilia are very distinct in Crocodiles; in these animals and Sphenodon the vertebral ribs have backwardly-projecting uncinate processes as in birds.