As mentioned on p. 278 the vertebrae of the Reptilia and those of all other Amniota are _gastrocentrous_; that is to say the centra or bodies of the vertebrae are formed by the pairs of interventralia, while the basiventralia are reduced, persisting either as so-called intercentra or wedge-bones, or as intervertebral pads, or disappearing altogether. At the earlier stages of development the gastrocentrous vertebrae behave in the {284}same way as that described on p. 12 (Fig. 1), except that the interdorsal elements are suppressed from the beginning. If the remaining three pairs of constituent elements of each vertebra (the basidorsalia, forming the neural arch; the interventralia, forming the body or centrum; and the basiventralia) remain separate, the vertebrae are called _temnospondylous_ (τέμνω, I cut, σπόνδυλος, a vertebra). If the neural arches and the centra are suturally united or are fused with each other, the vertebrae are called _stereospondylous_ (στερεός, solid). In many Amniota the atlas or first vertebra remains in a relatively primitive, embryonic condition, and is temnospondylous but for the usual modification that its centrum becomes attached to that of the second vertebra, and forms the odontoid process of the latter. The composition of gastrocentrous vertebrae (cf. p. 282) is best illustrated by the first and second cervical vertebrae of the Crocodile (Fig. 56, 3, p. 283). Concerning GEOGRAPHICAL DISTRIBUTION, even a cursory study shows that the sub-classes have come into existence at very different geological periods, and have each followed their own lines of dispersal. {285}CHAPTER VIII PROREPTILIA–PROSAURIA–THEROMORPHA _SUB-CLASS I.–PROREPTILIA._ _Permian Temnospondylous Reptiles with well-developed limbs and girdles of the terrestrial type._