VERTEBRAL 



COX^jN^K 

 bKK^^ ■ eacl 



199 



or in other words, two vertebJPiWT • each neuromere ^ (Pig. 

 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 * / \Ti.sp 



chordal sheath in CMmaera, 

 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 continuoiis 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 Fig. 114.— a, transverse section of the verte- 

 ,, . . ... T. ,. . bral column of Chimaera monstrosa ; B, 



their primitive distinctness. 1^^,^.^! ^j,,^, , ,._ Calcified ring ; h.r, basi- 



On the other hand, the basi- ventral; ««, inter-dorsal ; H.a, neural arch 



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



dorsals are much better Clevel- chordal sheath ; n.sp, neural spine (supra- 



oped than the inter-dorsals, dorsal). (From Parker and Haswell, after 



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 ; Eidewood, Journ. Linn. Soc. Zool. xxvii. 1899, p. 46. 



2 Gunther, Phil. Trans. 161, 1872, p. 526 ; Wiedersheim, Morph. Studien, Jena, 

 1880, Pt. i. p. 65 ; Gadow, op. cit. p. 198. 



