DISTINCTIVE CHARACTERS 
5 21 
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 ( x \j liat. 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 intercalaiy 
cartilages already alluded to are ovoid or diamond-shaped structures occurring 
