VERTEBRAL COLUMN 



199 



n.sp 



or in other words, two vertebrae to each neuromere l (Fig. 

 Ill, B). 



The Holocephali have a vertebral column essentially similar 

 to that of other Elasmobranchs, but of a more primitive type (Fig. 

 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. 

 Eibs 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 2 have much in Q 

 common with the Elasmo- 

 branchs, especially with cer- 

 tain of the acentrous Palaeo- 



h.r 



zoic representatives of that 

 group. The notochord is per- 

 sistent, centra are wanting, 

 and the different vertebral 



components Continue to retain FIG. 114. A, transverse section of the verte- 

 bral column of Chimaera monstrosa ; B, 

 lateral view, c.r. Calcified ring ; h.r, basi- 

 ventral ; int, inter-dorsal ; n.a, neural arch 

 (basi - dorsal) ; nch, uotochord ; nch.sh, 

 chordal sheath ; n.sp, neural spine (supra- 

 dorsal). (From Parker and Haswell, after 



Basse.) 



their primitive distinctness. 

 On the other hand, the basi- 

 dorsals are much better devel- 

 oped than the inter-dorsals, 

 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. i. p. 65 ; Gadow, op. cit. p. 198. 



