VIII VERTEBRAL COLUMN 199 
or in other words, two vertebrae to each neuromere! (Fig. 
wal, B). 
The Holocephalh have a vertebral column essentially similar 
to that of other Elasmobranchs, but of a more primitive type (Fie. 
114). The notochord is persistent and there are no centra; but ring- 
like calcifications, four or five 
to each neuromere, occur in the 
chordal sheath in Chimaera, 
although not in Callorhynchus. 
Ribs are absent. In the whip- 
like terminal portion of the 
tail the arcualia and the 
notochord become replaced by 
a slender continuous filament 
of cartilage. 
In the more obvious fea- 
tures of vertebral structure 
the Dipnoi? have much in 
common with the Elasmo- 
branchs, especially with cer- 
tain of the acentrous Palaeo- 
zoic representatives of that 
group. The notochord is per- 
sistent, centra are wanting, 
and the different vertebral 
components continue to retain Fic. 114.—A, transverse section of the verte- 
near ST aetimenies bral column of Chimaera monstrosa ; B, 
1e1r primitive distinctness. lateral view. c¢.7, Calcified ring ; h.7, basi- 
On the other hand, the basi- ventral; int, inter-dorsal ; 7.a, neural arch 
: (basi- dorsal) ; xch, notochord ; neh.sh, 
dorsals are much better devel- chordal sheath ; 7.sy, neural spine (supra- 
oped than the inter-dorsals, a (From Parker and Haswell, after 
asse. 
which are either vestigial or 
absent. The basi-dorsals unite in pairs over the spinal cord to 
form complete neural arches, and each arch supports dorsally 
the legs of a A-shaped, gable-like element or neural spine, which 
probably represents a pair of fused supra-basidorsals. Ventrally, 
there are basi-ventral cartilages, fused in pairs beneath the noto- 
chord, and supporting well-developed, bone -ensheathed ribs. 
1 Gadow, op. cit. p. 194; Ridewood, Journ. Linn. Soc. Zool. xxvii. 1899, p. 46. 
* Giinther, Phil. Trans. 161, 1872, p. 526; Wiedersheim, Morph. Studien, Jena, 
1880, Pt. 1. p. 65; Gadow, op. cit. p. 198. 
