5 3 2 
SHARKS AND RAYS. 
Of the numerous fossil genera of the family only a very brief 
yp mention can be made. One of the earliest is the Carboniferous 
Orodus, with teeth very like those of the later Hybodus, ranging from the Trias 
to the lower Cretaceous. In the last-named genus the notochord is persistent, the 
bluntly conical or cusped teeth have a central and two or more lateral cusps, the 
tin-spines are ridged, and there are two hook-like spines below each eye. Acrodus, 
with a nearly similar range, has, on the other hand, blunt teeth; while the Jurassic 
Aster acanthus differs from Hybodus by its rhomboidal, roughened, and flattened 
teeth, and the star-like ornamentation of the spines of the dorsal fins. In 
Synechodus of the Chalk all the teeth are cusped; the anterior ones having a tall 
central cusp, flanked with from three to five small lateral pairs. An allied extinct 
family (Gochliodontidce), confined to the Carboniferous rocks, differs by the 
component teeth of at least one of the oblique rows being fused -into a continuous 
curved plate, which may be either smooth or ridged. Many of the extinct 
representatives of these families exceeded the Port Jackson shark in size. 
The Comb-Toothed Sharks,—F amily No tidanidjE. 
LOWER TEETH OF EXTINCT COMB-TOOTHED SHARKS. 
■ A very remarkable family is now represented by the four species of comb- 
toothed sharks ( Notidanus ) and the frill-gilled shark ( Chlamydoselache anguineus), 
the latter of which is shown in the illustration. Whereas in all other sharks the 
gill-clefts are four in number, in the present family they are increased to five or six; 
while there is a further 
peculiarity in regard to the 
structure of the skull. It has 
been already stated (p. 520) 
that in the more typical 
sharks the functional lower 
jaw is articulated to the 
cranium by the intervention of the hyomandibular element; but in the Port 
Jackson shark this element becomes reduced in size, and the palatopterygoid bar 
(the functional upper jaw) has a facet by which it articulates directly with the 
cranium. In the present family, on the other hand, the hyomandibular takes no 
share in the suspension of the jaws, which are articulated to the cranium solely 
by means of the facet on the palatopterygoid bar; the latter joining a corre¬ 
sponding facet on the cranium behind the socket for the eye. In addition to their 
more numerous gill-slits, the comb-toothed sharks are distinguished externally 
from all those hitherto considered by having only a single dorsal fin, which is 
situated far back on the body and has no spine. The eye is devoid of a nictitating 
membrane; the spiracles are small; and the teeth, of which several series are in 
use at the same time, have sharply-pointed cusps. 
In the typical genus the body is moderately elongated, the 
mouth inferior in position, and the gill-openings, which may be either 
six or seven in number, are devoid of flaps. The principal teeth consist of a series 
of cusps placed upon a long base, all inclining in one direction, and decreasing in 
size from the front to the back; the number of these cusps being greater in the 
Typical Genus. 
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