VIII 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 Crossopterygi, 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 
Chondrostei1 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! 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 (M/yzine) 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. 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. 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 
! Schneider, Beitr. z. vergl. Anat. u. Entwickl. Wirbelth., Berlin, 1879, p- 51; also 
Gadow, op. cit. p. 196. 
2 Smith Woodward, Prit. Mus. Cat. Fossil Fishes, Pt. i. 1889, p. Xvi. 
