Little is at present known of the development of the vertebral column in either of the only two existing genera of Crossopterygii, _Polypterus_[184] and _Calamichthys_, and hence the precise mode of grouping of their vertebral components to form vertebrae is unknown. The condition of the vertebral column in the fossil forms varies greatly in different families, but in none is it so specialised as in the surviving members of the group. In the Devonian Holoptychidae, and even in genera so comparatively recent as the Upper Cretaceous Coelacanth _Macropoma_, the persistence of the notochord and the absence of centra indicate a very primitive grade of vertebral evolution. The Devonian and Carboniferous Rhizodontidae (e.g. _Eusthenopteron_ and _Rhizodus_), on the contrary, seem to have had well-ossified ring-like vertebrae. {203}In the caudal region of _Amia_ the basi-dorsals and basi-ventrals, and the inter-dorsals and inter-ventrals, form separate arch-centra which remain distinct; hence each vertebra is double, and there is a regular alternation of arch-bearing "pre-centra" and arch-less "post-centra" (Fig. 117, D). In the trunk-region the pre- and post-centra have fused, and in this region the vertebrae are single. [Illustration: FIG. 117.—A, precaudal vertebrae of _Caturus furcatus_; B, similar vertebrae of _Eurycormus speciosus_; C, caudal vertebrae of the latter species; D, caudal vertebrae of _Amia calva_. _h.a_, Haemal arch; _h.sp_, haemal spine; _hy.c_, hypo-centrum; _n.a_, neural arch; _n.sp_, neural spine; _p_, parapophysis; _p.c_, pre-centrum; _pl.c_, pleuro-centrum; _pt.c_, post-centrum; _r_, rib. (After Zittel.)]