200 FISHES CHAP. 
Inter-ventrals appear to be absent. Each neuromere corresponds 
with a pair of basi-ventrals, of basi-dorsals and of inter-dorsals. 
The haemal arches and spines are formed partly by the basi- 
ventrals, but mainly by the ventral union of the successive pairs 
of ribs. As in the Holocephali, the terminal arcualia of the tail 
become fused into a straight axial cartilaginous filament, trans- 
versely divided into segments, which replaces the notochord. 
nt.s -----f | < 
72 ; 
a | 
b. v . al | 
eS 
CUZ 
z Za 
Fig. 115.—Side view of the precaudal vertebrae of a Sturgeon (Acipenser sturio). a.c, 
Aortic canal, formed by the median union of ingrowths from the basi-ventrals and 
inter-ventrals of opposite sides; 6.d, basi-dorsals; 6.v, basi-ventral ; 7.d, inter- 
dorsal ; 7.v, inter-ventral ; , notochord ; v.c, neural canal; #.sp, neural spine ; 
nt.s, cuticular sheath of the notochord ; p, parapophysis ; 7, rib; s.n,° aperture for 
the root of a spinal nerve. 
Each segment supports a variable number of dorsal and ventral 
gable-pieces, or neural and haemal spines. Certain of the verte- 
bral components, such as the ribs, and the neural and haemal 
spines, are ensheathed by membrane bone. 
With certain modifications in details the preceding descrip- 
tion will also apply to the vertebral column of the Chondrostei 
(Fig. 115). It will be noted, however, that the inter-dorsals 
are much better developed than in the Dipnoi, although when 
