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.