VIII 



VERTEBRAL COLUMN 



203 



In the caudal region of Amia the basi- dorsals and basi- 

 ventrals, and the inter-dorsals and inter-ventrals, form separate 

 arch -centra which remain distinct; hence each vertebra is 

 double, and there is a regular alternation of arch-bearing " pre- 

 centra " and arch-less "post-centra" (Fig. 117, D). In the 

 trunk-region the pre- and post-centra have fused, and in this 

 region the vertebrae are single. 



A very primitive type of vertebral column occurs in some of 

 the Jurassic allies of Amia, in which certain of the vertebral 

 components, confluent in the adult Amia, retain some measure 

 of their primitive distinctness. 1 Thus, in the precaudal region 

 of Eurycormus (Fig. 117,B) there is a series of alternating dorsal 



FIG. 117. A, precaudal verte- 

 brae of Caturus fnrcatus ; 

 B, similar vertebrae of 

 Eurycormus speciosus ; C, 

 caudal vertebrae of the 

 latter species ; D, caudal 

 vertebrae of Amia calva. 

 h.a, Haemal arch ; h.sp, 

 haemal spine ; hy.c, hypo- 

 centrum ; n.a, neural arch ; 

 n.sp, neural spine ; p, para- 

 pophysis ;p.c, pre-centrum ; 

 pl.c, pleuro - centrum ; pt.c, 

 post-centrum ; r, rib. (After 

 Zittel.) 



and ventral half-rings of bone, which between them completely 

 invest the persistent notochord. Each ventral half -ring or 

 '' hypocentrum " represents a pair of fused and ossified basi- 

 ventrals, and possibly also a pair of included inter-ventrals, and 

 supports dorsally a pair of basi-dorsals, forming a neural arch, 

 and laterally a pair of ribs. The dorsal semi-rings, or " pleuro- 

 centra," similarly represent fused and ossified pairs of inter- 

 dorsals. In the tail, modifications approximating to what is 

 seen in the caudal region of Amia are to be noticed (C). By 

 the upgrowth of the ventral arch-bearing semi-rings, and their 

 conversion into complete rings encircling the notochord, incipi- 

 ent pre-centra are formed, and by a similar modification of the 



1 Zittel, Handb. d. Palae&ntologie, iii. 1887-1890, p. 137 et seq. ; Gadow, op. 

 cit. p. 208. 



