584 CHORDATA. 
Family 2.—Trichechidz.—The walruses ( 77ichechide) are Arctic 
and of large size. The teeth are blunt and reduced in number, the 
adult dentition being #428. 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.—Phocidz.—The seals ( Phoctde) have no pinnz 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. 
OrvDER XIII.—Jnsectivora. 
The mole is a member of this order and has been 
described as illustrating the fossorial or burrowing habit. 
As implied in the name, the /ysectivora are all feeders upon 
insects, worms and other small Znvertebrata. 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 Jusectivora 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 3443 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 Jvsectivora and 
Rodentia are therefore quite distinct. 
In the limbs the Zzsectivora 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 
