. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 530 Biology of the Vertebrates The amphicoelous type is found in most fishes, in primitive amphibians (stegocephalians) and primitive reptiles (cotylosaurs) as well as some living amphibians (, Proteus and Necturus) and reptiles (Sphenodon, some lizards and, to a certain extent, turtles). As mentioned previously the centrum of such a vertebra is concave at both ends (Fig. 432). Owing to the fact that the actual contact is limited to the edges of the tw
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 530 Biology of the Vertebrates The amphicoelous type is found in most fishes, in primitive amphibians (stegocephalians) and primitive reptiles (cotylosaurs) as well as some living amphibians (, Proteus and Necturus) and reptiles (Sphenodon, some lizards and, to a certain extent, turtles). As mentioned previously the centrum of such a vertebra is concave at both ends (Fig. 432). Owing to the fact that the actual contact is limited to the edges of the two cups, placed rim to rim, little freedom of movement is possible between the verte- brae held in place by considerable connective tissue, hence the limitation of this type mainly to water-dwellers. In a procoelous vertebra the cavity on the anterior end (pro, before) is retained but the posterior end fills in and becomes convex (Fig. 429). There is, therefore, a simple type of ball-and-socket joint which allows consider- able movement because of the reduced amount of connective tissue needed to lace together the vertebrae. Procoelous vertebrae occur in some anurans, chiefly frogs and toads, in the extinct pterosaurs, and in modern reptiles, including Lacertilia, Ophidia, and Crocodilia. The somewhat rare condi- tion of a biconvex vertebra is found in the case of the sacral region of Bujo and Rana and the first tail-vertebra of Crocodilia. The opisthocoelous condition, the reverse of the procoelous, has the concavity on the posterior end (opisthos, behind). These vertebrae are not characteristic of any major group but are found in widely separated species or Orders among all vertebrate Classes except cyclostomes. Examples are: Lepidosteus, the exception to the amphicoelous con- dition typical of fishes; some anurans; dinosaurs J-^â/^\ ^^p^y^ (cervical vertebrae only) ; penguins, parrots, and a 'Tr^M. x5?--»*J\ few otrier carinate birds; and ungulate mammals (cervical only).
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