192 BRITISH MAMMALS 



seals or sea lions than to the True Seals (Phocid^e). The walrus 

 is able to turn the hind feet forwards, as in normal mammals, for 

 purposes of locomotion. It also agrees with the Otariids in 

 having the skin of the feet prolonged into lobes beyond the 

 termination of the toes, and also in the very feeble nails on the 

 fingers of the fore paws. The claws on the hind feet are 

 proportionately longer than in the Otariids. In the base of the 

 skull there is an alisphenoid canal as in the sea lions, and there 

 is a small postorbital process. The testes, however, do not 

 descend into a scrotum. The nostrils are dorsal in position, and 

 more like those of the True Seals. From both sea lions and True 

 Seals the walruses differ markedly in their dentition. In the first 

 place, in the very young animal there are three pairs of minute 

 incisors in both jaws, an archaic feature lost apparently even in 

 the milk dentition of the Otariids and the Phocids. But in other 

 respects the teeth of the walrus show an extraordinary specialisa- 

 tion. In the adult animal the grinding teeth are practically 

 reduced to three pairs of premolars in each jaw, and these are 

 simple, rounded teeth without any distinct cusps. The incisors 

 are reduced to a single functional pair in the upper jaw, and 

 disappear altogether in the lower jaw, while the lower canine is a 

 small, round, blunt tooth like the premolars. Sometimes a fourth 

 premolar tooth of extremely minute size persists in the lower jaw, 

 and an equally minute fourth premolar and first molar in the 

 upper jaw. But the upper canine teeth are developed into a pair 

 of enormous tusks, longer by far even than the huge canine tusks 

 of the sabre-toothed Machairodonts. Just as the elephant is the 

 mammal which has developed incisor teeth most extravagantly, 

 so the walrus is the mammal which has carried furthest the 

 development of the canine teeth. The longest tusks as yet 

 obtained from this animal measure 31 in., of which about 

 24 in. would have protruded from the gum. The canine 

 tusks of the female are proportionately shorter. The longest 

 pair of female tusks obtained did not exeeed 20 in. in length. 

 In the Pacific Ocean, however, the walrus is said to develop even 

 longer tusks than those above recorded. 



