VERTEBRATA: MAMMALIA. 



737 



small apertures, which the animal has the power of closing 

 when under water. The bones are light and spongy, and 

 beneath the skin is a layer of fat or blubber. The dentition 

 (fig. 426) varies, but teeth of three kinds are always present, in 



Fig. 426. Dentition of the common Seal (Phoca vitulina). 



the young animal at any rate. The canines are always long 

 and pointed, and the molars are generally furnished with 

 sharp cutting-edges. The lower incisors may be reduced to 

 four or to two in number, or may even be wanting (Walrus) ; 

 and the upper incisors may fall below the normal six. The 

 dental formula of the common Seal (fig. 426) is 



3 3 i 



4 4 i 



The section Pinnigrada includes the three families of the 

 Earless Seals (Photidce), the Eared Seals (Otariadcz), and the 

 Walruses ( Tricheddce). 



The typical Seals (Phocidce] are distinguished from the Walruses by the 

 presence of incisor teeth in both jaws, and by canines of moderate size ; 

 while the absence of ears and the inability to use the hind-limbs on land 

 separate them from the Otariadce. They form a veiy numerous family, of 

 which species are found in almost every sea out of the limits of the tropics. 

 They abound, however, especially in the seas of the Arctic and Antarctic 

 regions. They live for the most part upon fish, and when awake, spend 

 the greater part of their time in the water, only coming on land to bask 

 and sleep in the sun and to suckle their young. They appear to be univer- 

 sally polygamous. The body is covered with a short fur, interspersed with 

 long bristly hairs ; and the lips are furnished with long whiskers, which 



3A 



