THE SB ARKS. 146 



Order 1. Plagiostomi. — Our most common shark is the 

 mackerel shark [Isunis punctatus, Fig. 186). It is from 

 four to eight feet in length, and is often taken in fish-nets, 

 being a snrface-swimmer. In the thresher shark [Alopecias 

 vulpes), 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 maxima, of the North Atlantic, which be- 

 comes nine to thirteen meti'es (thirty or forty feet) in 

 lengtli. It has very large gill-slits, and is by no means as 

 ferocious as most sharks, since it lives on small fishes, and 



FiQ. 187.— Carcharias. From Ltttken's Zoology. 



in part, probably, on small fl.oating animals, straining them 

 into its throat through a series of rays or fringes of an elas- 

 tic, 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), 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), which is a little larger than the dog-fish, 

 becoming over a metre (four feet) long, brings forth its 

 young alive. 



The hammer-headed shark is so called from the head 

 projecting far out on each side, the eyes being situated m 

 the end of each projection. 



