584 CHORD ATA, 



Family 2. — Trichechidae. — The walruses (TrichechidcB) are Arctic 

 and of large size. The teeth are blunt and reduced in number, the 

 adult dentition being W\%. The canines are long, forming the tusks : 

 they grow for some time from persistent pulps. The condition of the 

 teeth is correlated with the molluscan diet. As in the sea-lion, the 

 walrus can use its hind-limbs for terrestrial locomotion. 



Family 3. — Phocidae. — The seals ( Phocidce) have no pinnse to the 

 ears and the hind-limbs are permanently bent backwards. Hence the 

 seals are more exclusively aquatic than the preceding families. The 

 teeth are of the typical carnivorous type, with cusped ridged molars. 



Order XIII. — Insectivora. 



The mole is a member of this order and has been 

 described as illustrating the fossorial or burrowing habit. 

 As irrtplied in the name, the Insectivora are all feeders upon 

 insects, worms and other small Invertebrata. This diet 

 must of necessity be much more primitive than that of the 

 Carnivora or the Ungulata, for the invertebrate animals are 

 antecedent in time to the warm-blooded animals which 

 constitute the food of the former and to the grasses 

 devoured by the latter. Hence the Insectivora appear to 

 retain many dental features in common with the early 

 Eocene mammals. Their small size and general habits are 

 also usually of the primitive terrestrial type, though as in 

 all primitive groups certain members are very specialised 

 for particular habits. They are all diphyodont and hetero 

 dont, the molars are usually sharp-cusped and of the tri- 

 or quadri-tubercular types. On the whole, the dentition 

 most resembles that of certain. Carnivora, but the canines 

 are never so prominent as in this order. The typical 

 Eutherian dentition of Ijff is common. In external 

 appearance a number are closely similar to the Rodentia, 

 but they never possess the peculiar incisor teeth of this 

 order. There are always more than two pairs of incisors 

 on each side of the lower jaw and they do not grow from 

 persistent pulps. The dental characters of Insectivora and 

 Rodentia are therefore quite distinct. 



In the limbs the Insectivora are little modified from the 

 mammalian type. There are five digits on each limb and 

 they are plantigrade \ in these respects they differ from a 

 great number of Carnivora, but in addition they nearly all 



