PORPOISES, DOLPHINS, AND WHALES 25 



most nearly to the fish-like form. The limbs, too, are modified 

 into webbed paddles, the hinder pair of which are fairly free in 

 the Sea- Lions and Walruses, but in the Seals are backwardly 

 directed and bound up by skin with the tail so as to form a 

 powerful propeller, eminently adapted for rapid swimming, but 

 quite unsuited for movement on land, so that while a walrus or 



Fig. 318. Skulls of Seal (A) and Walrus (B) 



sea-lion can shuffle along fairly well on the ice, a seal 

 has much greater difficulty in getting along when re- 

 moved from the water. The teeth of these marine 

 Carnivores are in a very interesting condition, having 

 a direct relation to their food. Sea- Lions and Seals 

 live chiefly on fish, and possess numerous sharply- 

 pointed teeth, well adapted for seizing and holding such slippery 

 prey (fig. 318). Walruses mainly subsist on burrowing shell-fish 

 and other creatures living in mud or sand, and they have a pair of 

 long, tusk-like canines in the upper jaw, by which their food can 

 be dug out of the sea-bottom (fig. 318). These tusks continue 

 to grow throughout life, and in accordance with this do not taper 

 into fangs within the jaw, but project far beyond it. The re- 

 maining teeth are blunt and simple, their form fitting them for 

 crushing the food. 



PORPOISES, DOLPHINS, AND WHALES (CETACEA) 



The Mammals included in this class have become very much 

 specialized to fit them for an aquatic life, and, although their 

 remote ancestors were undoubtedly terrestrial, they have no very 

 near allies among existing land animals. The body is fish-shaped, 

 and suited for rapid progression through the water, propelled by 

 the powerful tail, which is broadened out horizontally (not verti- 

 cally, as in a fish). The fore-limbs are paddle-like flippers, without 

 external trace of digits, and the hind-limbs are only represented (in 



