xxvi i. 7 SEALS 691 



prised that the palaeontologist, who has to consider also variations 

 with time, finds that the growth of his material introduces intolerable 

 problems of classification. 



7. Suborder Pinnepedia 



The seals, sea-lions, and walruses are marine carnivores that have 

 existed since the Miocene. Their exact affinities are doubtful; they 

 show some similarities to otters, perhaps due to common ancestry 

 but possibly to convergence. They also have a remote likeness to the 

 bears. 



Fig. 463. Skeleton of the seal, Phoca. (After Blainville.) 



In the seals (Phocidae) the body is streamlined, covered with thick 

 fur, below which is a thin epidermis and a thick layer of blubber, 

 making a quarter of the weight of the animal. Swimming is by means 

 of the paddle-like limbs and flexion of the whole body, there is web- 

 bing between the digits, and the tail is reduced to a short rudiment. 

 The basal segments of the limbs are shortened and some of the digits 

 lengthened, without any increase in their number, though there are 

 some extra phalanges. The speed of swimming may reach 15 knots if 

 the seal is frightened. The cervical vertebrae are massive, with com- 

 plex articulations, but the hinder ones are simplified and the column 

 very flexible, so that it can be bent dorsally or laterally, allowing 

 sudden turns in the water and complicated balancing feats on land 

 (Fig. 463). All the seals leave the water to breed and therefore need 

 some support for their large bodies. 



The teeth show a reduction in number and are rows of nearly 

 similar, laterally compressed spines. They may carry three cusps in 

 a row, a reversion to 'reptilian' conditions, which serves to prevent 

 escape of the slippery prey. There are large canines. The milk denti- 

 tion is lost very early, sometimes in utero. The intestine is long. The 

 water-supply is obtained from metabolic water. 



