SELACHOIDEI. 



317 



tropical, but less so in the temperate seas. Bet-v(reen thirty 

 and forty different species have been distinguished, of which 



Fig. 112. — Dentition of the Blue Shark (Carcharias glaucus) ; 

 the single teeth are of the natural size. 



one of the most common is the "Blue Shark" {Carcharias 

 glaucus). Individuals of from twelve to fifteen feet are of 

 very common occurrence, but some of the species attain a 

 much larger size, and a length of 25 and more feet. Fishes 

 of this genus or of closely allied genera [Corax, Hemipristis) 

 are not uncommon in the chalk and tertiary formations. 



Galeooerdo. — Teeth large, flat, triangular, oblique, serrated 

 on both edges, with a deep notch on the outer margin. Spiracles 

 small. A pit on the tail, above and below, at the root of the 

 caudal fin. Two notches on the under caudal border, one of 

 them at the end of the spine. 



Three species, of which one {G-. arcticus) is confined to 



