Vul VERTEBRAL COLUMN 203 
In the caudal region of Amia the basi-dorsals and basi- 
ventrals, and the inter-dorsals and inter-ventrals, form separate 
areh-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 Ama, in which certain of the vertebral 
components, confluent in the adult Ama, retain some measure 
of their primitive distinctness." Thus, in the precaudal region 
of Eurycormus (Fig. 117, B) there is a series of alternating dorsal 
Fic. 117.—A, precaudal verte- 
brae of Caturus furcatus ; 
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 ; 7.a, neural arch ; 
n.sp, neural spine ; p, para- 
pophysis ; p.c, pre-centrum ; 
pl.c, pleuro-centrum ; pit.c, 
post-centrum ; 7, 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 
! Zittel, Handb. d. Palacontologie, iii. 1887-1890, p-. 137 et seq. ; Gadow, op. 
cut. p. 208. 
