2896 



SHARKS AND RAYS 



usually communicate with the exterior by means of five vertical slits on the sides of 

 the neck, although occasionally the number of these clefts is increased to six or 

 seven. Very generally the mouth is situated on the inferior aspect of the head; and 

 the teeth carried on the functional jaws may be either sharply-pointed and separate, 

 or blunt and articulated, so as to form a more or less pavement-like structure. 



HAMMER-HEADED SHARK. 



(One-tenth natural size.) 



In the former case there is a continuous succession of new teeth to replace the old 

 ones as they are worn away and shed. As a rule, the tail fin is heterocercal, with 

 the upper lobe greatly elongated; the pelvic fins are always abdominal in position; 

 and the dorsal fins of many extinct and a few living types bear large spines on their 

 front edge, which, unlike those of the bony fishes, are simply imbedded in the flesh, 

 without articulating with the internal skeleton, and are consequently immovable. 



