108 VERTEBRATA. MAMMALIA. 



nostrils are closed at will; the orifice of the ear 

 can also be closed ; and the eye is furnished with a 

 third eyelid, which can be drawn across it as a 

 defence. The limbs are short, and almost enclosed 

 within the body, little more than the feet projecting : 

 the toes, however, being strongly webbed, render 

 them very efficient as fins : and the hind pair, in 

 particular, from their shape and position, have 'a 

 strong resemblance to the broad horizontal tail of 

 the Cetacea. In fact they answer the same purpose 

 that of sending up the animal rapidly to the surface, 

 by a powerful vertical stroke. The feet are scarcely 

 used in motion on land, where yet they move with 

 considerable speed. The mode in which this is 

 effected, is another of the interesting peculiarities 

 of this tribe. The vertebrae of the spine are much 

 more separated than usual, the connecting cartilages 

 being much larger, allowing it great freedom of 

 curvature, and the muscles which bend it are par- 

 ticularly strong. In moving forward the Seal arches 

 its spine, bringing the hinder part forward, then pres- 

 sing with the hind feet on the ground, straightens 

 the body with a jerk ; and by a repetition of such 

 apparently awkward springs as these, it manages 

 to get along at a good round pace. 



There is some reason for believing that vegetables 

 occasionally form the food of the true Seals ; but 

 their dentition is decidedly carnivorous, the molars 

 being all trenchant, or conical, without any tuber- 

 culous part at all. The Walrus, on the other hand, 

 the connecting link with the Herbivorous Mammalia, 



