532 SHARKS AND RAYS. 



Of the numerous fossil genera of the family only a very brief 

 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 

 fin-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 

 Asteracanthus 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 (Cocldiodontidce), 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, Family NOTIDANID^E. 



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 



LOWER TEETH OF EXTINCT COMB-TOOTHED SHARKS. sharks tll6 functional loWei* 



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, whicli 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. 



T ical Genus "^ n ^ e typical g en ^s 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 



