{283}[Illustration: FIG. 56.–Composition of vertebrae of Reptiles, illustrated by the first and second cervical vertebrae. (1) Atlas (first cervical) and axis (second) vertebra of _Crocodilus_. (2) Atlas and axis of _Metriorhynchus_, a Jurassic Crocodile. (3) Analysis of the first two cervical vertebrae of a Crocodile; 2, second basiventral complex or "intercentrum" continued upwards into the meniscus or intervertebral pad. (4) Diagram of the fundamental composition of a Reptilian vertebra; compare this and (6) with Fig. 1 (8 and 9) on p. 13. (5) The first three cervical vertebrae of _Sphenodon_. (6) Trunk-vertebrae of _Eryops_, a Permian Proreptile; typically temnospondylous; _cp_, articular facet of the capitulum of a rib. (7) The complete atlas of an adult _Trionyx hurum_; the second basiventral (intercentrum) is attached to the posterior end of the first centrum, which, not being fused with the second centrum, is not yet an odontoid process. (8) The complete atlas of an adult _Trionyx gangeticus_; still typically temnospondylous. (9) The first and second cervical vertebrae of an adult _Platemys_. (10) The complete atlas of a _Chelys fimbriata_. _Az_, Anterior zygapophysis; _B.D_, basidorsal; _B.V_, basiventral; _C_{1}_, _C_{2}_, _C_{3}_, first, second, and third centra, formed by the interventralia; _Cp^1_, _Cp^2_, articular facets of the capitular portions of the first and second ribs; _I.V_, interventral; _N_{1}_, _N_{2}_, _N_{3}_, first, second, and third neural arch, formed by basidorsalia (_B.D_); _Od_, odontoid process = first centrum; _Pz_, posterior zygapophysis; _R_{1}_, _R_{2}_, ribs; _Sp_, detached spinous process of the first neural arch; _t_{1}_, _t_{2}_, tubercular attachments of the first and second ribs; 1, 2, 3, 4, "intercentra" = basiventrals; _I_, _II_, _III_, position of the exit of the first, second, and third spinal nerves.]