vni VERTEBRAL COLUMN 197 



The independent evolution of a more specialised vertebral 

 column from a more primitive one may often be traced within 

 the limits of the same group of Fishes when the more ancient 

 genera are compared with the more recent. In the Elasmo- 

 branchs and the Crossopterygii, for example, the oldest known 

 types were acentrous, while the more recent have acquired calci- 

 fied or bony centra, and altogether they have reached a more 

 advanced stage of vertebral evolution. Some Fishes, like the 

 Chondrostei and the Dipnoi, seem, however, to exhibit com- 

 paratively little advance in vertebral structure, since both the 

 Palaeozoic, and the living representatives of these groups agree 

 in being acentrous. 



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. 1 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 Elasmo- 

 branchs, 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. Hylodus) 

 were also devoid of centra, at least in the trunk-region. The 

 first indication of complete centra occurs in the Low r er Lias 

 Cestraciont, Palaeospinace. 2 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 



1 Schneider, Beitr. z. vergl. Anat. u. Entwickl. Wirbelth., Berlin, 1879, p. 51 ; also 

 Gadow, op. cit. p. 196. 



2 Smith Woodward, Brit. Mus. Cat. Fossil Fishes, Ft. i. 1889, p. xvii. 



