The teeth of reptiles are composed of the same kinds of tissues as are the teeth of mammals, that is, of dentine and enamel, but the enamel is always thin, perhaps because the teeth are so easily replaced that a thicker protective covering is not needed. The arrangement of the dentine in primitive reptiles is complicated, that is, plicated or folded in labyrinthine figures, like that of many stegocephalian amphibians, the Labyrinthodontia, especially. This labyrinthine structure of the dentine persisted longest in the ichthyosaurs. VERTEBRAE AND RIBS The spinal column or backbone of reptiles, as in all air-breathing vertebrates, is made up of a variable number of separate segments called vertebrae, permitting flexibility. Each vertebra is composed of a body, or centrum, and an arch on the dorsal side for the protection of the spinal cord. Various projections from the vertebra, called processes, serve for the attachment of ligaments or muscles, for articular union with adjacent vertebrae, or for the support of ribs, and these processes have characteristic differences in different reptiles. The pair in front and behind, for articulation with the adjoining vertebrae, may become obsolete or even lost in swimming reptiles, as we shall see; they are called zygapophyses. In not a few reptiles there is an additional pair for zygapophysial articulation in front and behind, called zygosphene and zygantrum, for the greater strengthening of the column; they are especially characteristic of snakes and certain lizards. In certain other reptiles, especially the long-necked dinosaurs, there is an additional pair arranged differently from the zygophene, that have received the names hyposphene and hypantrum. [Illustration: FIG. 12.—Procoelous vertebra of snake: _za_, zygantrum; _zs_, zygosphene; _ps_, posterior zygapophysis.] On the top of the arch is the spine or spinous process, which may vary enormously in size and length; sometimes it is flattened or dilated above for the support of an exoskeleton, or it may be heavy and massive for the attachment of strong muscles and ligaments. In the modern basilisk lizards and in the ancient _Dimetrodon_ and _Edaphosaurus_ from the Permian rocks of Texas these spines are of enormous length, some of them nearly four feet long in reptiles not twice that length. Slender crawling reptiles usually have no spines, or only vestigial ones. On the sides of the arch there may be a distinct transverse process for the articulation of the rib.