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 Amert- 
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 levis Risso 3 
so-called placenta is developed, while it is wanting in the 
Mustelus vulgaris of Miller and Henle. 
