ORDERS OF MAMMALIA. 333 
2. Plantigrada (fig. 180, A), comprising the Bears, in which 
the whole, or nearly the whole, of the foot is applied to the 
ground, so that the animal walks upon the soées of the feet. 
3. Digitigrada (fig. 180, C), comprising the Cats, Lions, Tigers, 
Dogs, &c., in which the heel is raised from the ground, and the 
animal walks upon tip-toe. 
The Seals and Walruses, forming the family Pznnuigrada, are 
distinguished from the other Carnzvora by their adaptation to 
an aquatic mode of life. In this respect they agree with the 
thoroughly aquatic Whales and Dolphins, but they differ from 
both the Cetacea and the Szrezza, not only in their dentition, 
but also in always having well-developed Azna-limbs. The 
Seals (fig..181), are characterised by having incisor teeth in 
Fig, 181.—Greenland Seal (Poca Groenlandica). 
both jaws, at the same time that the canine teeth are not 
immoderately developed. They form a very numerous family, 
of which species are found in most seas out of the limits of the 
tropics. They abound, however, especially in the seas of the 
Arctic and Antarctic regions. They are largely captured both 
for their oil and for their fur. The Walrus or Morse (Z7rechecus) 
is distinguished from the true Seals by the fact that in the adult, 
only two of the upper incisors are present ; whilst the upper 
canines are enormously developed, and form two pointed tusks— 
fifteen inches or more in length—which are directed downwards 
between the small lower canines, and project considerably 
below the chin. The Walrus is a large heavy animal, from 
