DISTINCTIVE CHARACTERS. 521 



skin by their margins, and usually communicate with the exterior by means of 

 live 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 together, so as to 

 form a more or less pavement-like structure. In the former case there is a 



HAMMER-HEADED SHARK (^ nat. size). 



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. Spiracles 

 are frequently developed on the upper surface of the head ; and the intercalary 

 cartilages already alluded to are ovoid or diamond-shaped structures occurring 



