FISHES CHAP. 
194 
and ventral surfaces of the notochord, which, as they form 
portions of a system of dorsal and ventral arches, are termed 
“arcualia” (Fig.111). On the dorsal side there are: (1.) a series 
of paired basi-dorsal cartilages (neurapophyses or neural arches), 
the two elements of each pair contributing to form the side walls 
of the neural canal in which the spinal cord is lodged (Fig. 112, 
A): (ii) a series of inter-dorsal cartilages (intercalary neural 
arches), regularly alternating with 
the preceding, and completing 
the walls of the neural canal by 
filling up the intervals between 
the basi-dorsals ; and (iii.) a series 
of supra-dorsal elements, typi- 
cally also in pairs, but in the 
Dog-Fish fused to form single 
median cartilages. Of the latter 
there are two sets—one the 
supra-basi-dorsals, or neural 
spines, are situated over the basi- 
od. 
id. 3 
Fic. 111.—A, side view of precaudal verie- 
brae of Scyllium canicula ; B, similar 
view of caudal vertebrae. 6.d, Basi- 
dorsal ; c, centrum; A, basi-ventral ; 
h.s, haemal spine; i.d, inter-dorsal ; 
p, parapophysis; 7, rib; s.d, supra- 
dorsals. The vertical dotted lines 
indicate the limits of neuromeres and 
myotomes. The small circles represent 
the exits of the dorsal and ventral roots 
of spinal nerves. (After Ridewood. ) 
ment is substantially repeated 
dorsals; and the other, supra- 
inter-dorsals,alternating with the 
former, lie over the inter-dorsals, 
the two series forming the key- 
stones of the dorsal arches, and 
thus completing the roof of the 
neural canal. On the ventral side 
of the notochord this arrange- 
by a series of ventral arcualia, 
which, however, are somewhat differently arranged in the 
trunk and tail. Thus, in the trunk there are: (1.) a series of 
basi-ventral or haemal cartilages, corresponding with the basi- 
dorsals above, which grow out laterally into short processes, 
the parapophyses or transverse processes, and terminate in 
(ii.) short, slender cartilages—the costal elements or ris—which 
may perhaps be regarded as the ventral equivalents of supra-basi- 
dorsals. The ribs project outwards into the dorsal wall of the coelom 
and end in the myocommata separating the myotomes of the body- 
wall. In the tail the basi-ventrals lose their ribs and, growing down- 
wards into ventral prolongations, they unite in pairs beneath the 
caudal artery and vein, and so form a series of inverted arches 
