420 ZOOLOGY. 



or Isurus punctatus (Fig. 389). The head is conical, with 

 the nostrils under the base, and the lobes of the tail are 

 nearly equal. It is from four to eight feet in length, and is 

 often taken in fish-nets, being a surface-swimmer. In the 

 thresher shark (Alopecias vulpes Cuvier), the upper lobe of 

 the tail is nearly as long as the body of the shark itself. It 

 grows twelve or fifteen feet in length, and lives on the high 

 seas of the Atlantic. 



Nearly twice the size of the thresher is the great basking 

 shark, Selache (Cetorhinus) maxima Cuvier, of the North 

 Atlantic, which becomes nine to thirteen metres (thirty or 

 forty feet) in length. It has very large gill-slits, and is by 

 no means as ferocious as most sharks, since it lives on small 



Fig. 389. Mackerel Shark. From Tenney's "Zoology." 



fishes, and in part, probably, on small floating animals, strain- 

 ing them into its throat through a series of rays or fringes of 

 an elastic, hard substance, but brittle when bent too much, 

 and arranged like a comb along the gill-openings, the teeth 

 being very small. 



Among the smaller sharks is the dog-fish (Squalus Ameri- 

 canus Storer), distinguished by the sharp spine in front of 

 each of the two dorsal fins., It is caught in great numbers 

 for the oil which is extracted from its liver. The dog-shark 

 (Mustelus canis Dekay), which is a little larger than the 

 dog-fish, becoming over a metre (four feet) long, brings forth 

 its young alive. In the European Mustelus Icevis Risso a 

 so-called placenta is developed, Avhile it is wanting in the 

 Mustelus vulgaris of Mtiller and Henle. 



