PLAGIOSTOMI OE SELACHII : SHARKS. 



259 



Dog-fishes (Fig. 300) have the dorsal fins furnished with 

 strong spines. Similar fossil spines, more than two feet 

 in length, are found in the Devonian and Carboniferous 

 rocks. The Cestraciont Sharks, around Australia, have 

 the mouth furnished with a pavement of bony pieces, for 

 a masticating apparatus; the margins of their jaws, how- 



Thresher Shark, Alopias vulpes, Bon. 

 FIG. 303. 





White Shark. 



ever, have ordinary teeth. The Threshers (Fig. 302) have 

 the upper lobe of the tail excessively elongated. 



The eggs of Sharks and Skates are in the form of an 

 oblong sack with the corners greatly prolonged into strings, 

 and the covering is horn-like in consistency. The young 

 are much developed before the eggs are laid ; and in some 

 species the young are hatched in the body of the parent, 

 so as to make these fishes appear as true viviparous animals. 



