THE SHARKS. 
145 
Order 1. Plagiostomi. — Oar most common shark is the 
mackerel shark {Isiirus pwictatus, Fig. 186). 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), 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 tlie size of the thresher is the great basking 
shark, Selache maxima, of the North Atlantic, which be- 
comes 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 fishes, and 
Fig. 187— Car char ias. From Liitken's Zoology. 
in part, probably, on small floating 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 {Squalits 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 
{Micstelus ca7iis), 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 in 
the end of each projection. ^ 
