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.