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This group includes the Seals, Sea-Lions, and Walruses, all aquatic and, for the larger part, marine creatures. Being aquatic they have to some extent acquired a fish-like form, though not so completely as have the Whales and even the Sirenia. This is most complete so far as the group is concerned in the Seals, where the hind-limbs have become soldered to the tail and are inefficient as walking legs, where the external ears have vanished, and where the general shape of the body is tapering and thus fish-like. The Walruses and Sea-Lions are less modified in this direction; in the latter (not in the former) the external ear, though small, is persistent, and the hind-limbs are capable of being used as organs of progression upon dry land. The general characters applicable to the Carnivora, given upon a previous page, apply to the Pinnipedia.

The characters confined to the Pinnipedia as a whole are mainly these:—The greater part of the limbs are enclosed within the skin, the hands and feet are fully webbed, and there is a tendency for the nails to disappear, and for the phalanges to increase in number—characters which are clearly not diagnostic of the order but correlated with an aquatic life, since they reappear, and are indeed exaggerated, in the Cetacea. The teeth are peculiar in that the milk dentition is feeble and is early shed. This, as it were, undue emphasis upon one of the two sets of teeth is another likeness to the Whales,