204 FISHES CHAP. 
down-growing, archless, dorsal half-rings, structures comparable 
to post-centra are produced. In brief, Hurycormus, as well as 
such other extinct Amioid genera as Caturus (Fig. 117, A), Cal- 
lopterus, and Huthynotus, retain in the adult a stage of vertebral 
evolution which is closely paralleled by transitory stages in the 
embryonic and young forms of Ama. 
Lepidosteus' is unique amongst existing Fishes in having 
opisthocoelous vertebrae; that is, the centra are convex in front 
and concave behind, 
and therefore articu- 
late with one an- 
other by ball-and- 
socket joints (Fig. 
118). This condi- 
tion is due to the 
development of a 
series of interver- 
tebral rings of car- 
tilage round the 
notochord. The 
subsequent inward 
growth of each of 
these rings leads to 
the constriction, and 
ultimately to the 
complete — oblitera- 
cn tion, of the noto- 
Fic. 118.—A, two vertebrae from the trunk-region of chord, much in the 
Lepidosteus ; B, anterior face of a vertebra. CN, ‘ 
Anterior convex face of the centrum; c.n’, posterior SAIW€ Way as by 
concave face ; h.a, parapophysis, with its articular facet the orowth of ordi- 
for a rib; i.c, median cartilage, representing a pair of 2 
fused supra-interdorsals ; 7.s, radial element of the dorsal Nary centra. Later, 
fin ; 2.2, superior longitudinal ligament ; ».a, neural arch. j 201] 
(From Wiedersheim, after Balfour and W. N. Parker.) this ‘solid mae 
cartilage becomes. 
transversely divided by a cleft which is convex anteriorly and 
concave behind (Fig. 116, C), and of the two portions one fuses 
and co-ossifies with the centrum of the vertebra in front, and the 
other with the one pertaining to the vertebra behind. Reference 
to Fig. 116 will show that the grouping of the vertebral elements 
to form the individual vertebrae is not the same as in Amia. 
1 F. M. Balfour and W. N. Parker, Phil. Trans. 173, 1882, p. 388. 
