VIII VERTEBRAL COLUMN 201 
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,’ 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 
ID BD 
A tie 
Fic. 116.—Diagram to illus- 
BV IV trate the grouping of 
vertebral elements to form 
pc pt.c 
vertebrae, A, in an Elasmo- 
branch, B, in Ama, and 
C, in Lepidosteus. B.D, 
Basi-dorsals ; B.V, basi- 
ventrals; J.D, inter- 
dorsals ; J. V, inter-vent- 
rals; in.v.c, Inter-verte- 
bral cartilage divided by 
a concavo - convex ‘cleft ; 
p.c, precentrum; pt.c, 
postcentrum., The square 
blocks represent  indi- 
vidual vertebrae, and the 
oblique lines, the attach- 
ments of the myocommata. 
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.” 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. 
