Some of the more notable features in the structure of the vertebral column in the Cyclostomata and Fishes will now be briefly considered. In the Cyclostomata the acentrous vertebral column is more primitive than in any other Craniates, and in the Lamprey it consists of a persistent notochord, supporting a series of isolated cartilages on each side of the spinal cord.[177] As two pairs of these cartilages are included in each neuromere it is possible that they represent alternating basi-dorsals and inter-dorsals. There are no ventral arcualia in the trunk and no ribs. In the Hag-Fish (_Myxine_) the dorsal cartilages are restricted to the tail. The description of the vertebral column of the Dog-Fish may be taken as fairly applicable to Elasmobranchs in general, and hence only certain notable features in some other members of the group need be referred to here. The most primitive Elasmobranchs, the Palaeozoic genera _Cladoselache_ and _Pleuracanthus_ were acentrous, although calcified rings have been observed in a Permian species of the latter genus and scattered calcifications in others. Some of the earlier Mesozoic genera (e.g. _Hybodus_) were also devoid of centra, at least in the trunk-region. The first indication of complete centra occurs in the Lower Lias Cestraciont, _Palaeospinax_.[178] All the later extinct, as well as all existing forms, have more or less well-developed centra, hardened by the deposit of lime salts in their primitively cartilaginous substance, but never in the form of true bone. {198}[Illustration: FIG. 113.—Schematic transverse section through the middle of a Cyclospondylic (A), a Tectospondylic (B), and an Asterospondylic vertebra (C). _d_, Middle portion of the calcified double cone; _d′_, additional concentric calcified layers; _d″_, double cone with radiating calcified layers; _ex.m_, external elastic membrane; _h.a_, haemal arch; _n.a_, neural arch; _n.c_, notochordal cavity. (From Zittel, after Hasse.)]