2908 



SHARKS AND RAYS 



I.OWER TEETH OF EXTINCT COMB-TOOTHED SHARKS. 



gill clefts are five in number, in the present family they are increased to six or 

 seven; while there is a further peculiarity in regard to the structure of the skull. 

 It has been already stated (p. 2895) 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 Jack- 

 son shark this element be- 

 comes reduced in size, and the 

 palatopterygoid bar (the func- 

 tional 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 sus- 

 pension of the jaws, which are articulated to the cranium solely by means of the 

 facet on the palatopterygoid bar; the latter joining a corresponding 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 

 Typical Genus ..... ... , ,, ... . ... , . , 



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 



SHARK. 



from the front to the back; the number of these cusps being greater in the teeth 

 of the lower than in those of the upper jaw. With the occasional exception of 

 some portions of the tail, the notochord persists throughout life. At the present 

 day the range of the existing members of the genus includes most temperate and 



