VERTEBRAL COLUMN 



2OI 



compared with the basi-dorsals they take but a small share in 

 forming the walls of the neural canal. Well -developed but 

 somewhat fragmentary inter-ventrals are present. The haemal 

 arches and spines are formed by the downgrowth and ventral 

 union of the basi-ventrals as in the Dog-Fish, and apparently 

 without the aid of costal elements. In Polyodon the ribs are 

 vestigial, 1 but in Acipenser they are well developed. ' The neural 

 arches and spines, and their haemal representatives in the tail, 

 and also the ribs, are partially ossified, or ensheathed by bone. 

 In the existing Crossopterygii, Holostei, and Teleostei, popularly 



FIG. 116. Diagram to illus- 

 trate the grouping of 

 vertebral elements to form 

 vertebrae. A, in an Elasmo- 

 branch, B, in Amia, and 

 C. in Lepidostevs. B.D, 

 Basi-dorsals; B. V, basi- 

 ventrals ; I.D, inter- 

 dorsals ; /. V, inter-vent- 

 rals ; iti.v.c, inter- verte- 

 bral cartilage divided by 

 a concavo - convex cleft ; 

 p.c, precentrum ; pt.c, 

 postceutrum. The square 

 blocks represent indi- 

 vidual vertebrae, and the 

 oblique lines, the attach- 

 ments of the myocommata. 



ID 



BD 



BV 



ID X 



BD 



BV \!V 



known as the " bony Fishes," the vertebral column assumes a more 

 familiar character, and at the same time we meet with interesting 

 illustrations of the different methods by which the separate com- 

 ponent vertebral elements of the more primitive types of " back- 

 bone " are concentrated together in groups, and fused to form that 

 complex physiological product, the complete bony vertebra. 2 In 

 most of these Fishes each vertebra is formed by the aggregation 

 and fusion of a pair of basi-dorsals and a pair of basi-ventrals, 

 and includes, in addition, a pair of inter-dorsals, which may 

 either be the pair in front of the basi-dorsals or the pair behind, 

 and also a pair of inter-ventrals, which, similarly, may be the 



1 Bridge, P.Z.S. 1897, p. 722. 2 Gadow, op. cit. p. 201, et. seq. 



