No: 1.] MORPHOLOGY OF CLADOSELACHE. 95 
that even of the ventral fin of Xenacanthus (female). The 
axis of the basalia in the latter has already emerged from 
the body wall, has become of the archipterygial type, cluster- 
ing its radials distally in what Fritsch and Wiedersheim call 
the fost-axtal Strahlenreihe. This fin type will shortly (as 
in the pectoral of the same form) proceed to form the 
biserial archipterygium by the process of ray splitting being 
continued from the tip to the opposite side (pre-axial of 
Fritsch) of the fin axis. 
One would now naturally look to the simpler structure of 
the ventrals of Cladoselache for further light on the primitive 
vertebrate extremity. One of the best specimens of these has 
already been figured by Newberry,!! but may here (Fig. 2), 
with slight modifications, again be figured.! 
The ventral is first to be noted as a longitudinal fin nearly 
three times as long as wide, its length alone suggestive of its 
archaic fin-fold character. Its plane is that of the pectoral; it 
is more delicate, and smaller, less by two-thirds in linear 
measurements ; its position is midway between pectoral and 
caudal. Like those of pectoral its rays are unjointed, proceed 
from body-wall to fin-tip ; the foremost are the smallest, stoutest 
and most clustered. Unlike the rays of the pectoral they 
never branch, are distinctly and sharply tapering, show no 
specialized shapes, are directed somewhat backward, and vary 
but little in their angle of inclination, save that the posterior 
rays are directed slightly further backward. The ray concen- 
tration at the anterior margin is regularly accomplished. In 
the median portion of the fin nearly the entire length of the 
alternating rays, z.e., those whose bases have been compressed 
into the ventral fin-surface, is to be seen. The sloping, anterior 
fin-margin is exactly what might be expected if the intervening 
fin-fold be imagined to have slowly disappeared; its slow 
disappearance might also account for the concentration of the 
rays in the anterior margin, whose function now must include 
that of cut-water. The hind portion of the fin, naturally 
least modified, seems little more than the actual remnant of 
1 In no specimen are claspers present ; sex apparently is not to be distinguished 
by the shape or character of ventrals. 
