AQUATIC MAMMALS 



The remains of forms supposed to be intermediate between otters 

 and true mustelids are known from the Tertiary, but these are frag- 

 mentary, and nothing, of course, can be told regarding any aquatic 

 adaptations which they may have shown. 



Enhydrinae — the sea otter, of a single genus and species, with dis- 

 tribution in the north Pacific and south along the American coast into 

 the waters of Lower California. Apparently this animal spends its 

 entire time in the sea save occasionally when it may haul out on the 

 rocks to rest. It is said to give birth to the young upon floating patches 

 of kelp. External modifications for an aquatic life consist chiefly of a 

 reduction in the size of the external ear and of the fore feet, increase in 

 the size of the hind feet, which are fully webbed, with hairy sole and 

 with the fifth toe the longest, in the slight vertical flattening of the 

 moderate sized tail, and the excessive softness and denseness of the 

 pelage. Significant points in its osteology consist of reported increase 

 in the flexibility of the vertebral column, slight shortening of the neck, 

 flaring of the anterior element of the pelvis, slight shortening of the 

 fore limbs and of the thigh and shank as well, possible slight increase 

 in stoutness of the hind limb and enlargement of the pes. 



In spite of my best eff^orts and numerous letters to those who should 

 know, I have been entirely unsuccessful in ascertaining the precise move- 

 ments of the hind feet whereby this animal accomplishes swimming. 

 Possibilities will, however, be discussed in chapter eleven. 



PINNIPEDIA 



All the pinnipeds are highly aquatic, to only a lesser degree than the 

 whales and sirenians, and are chiefly pelagic. As in the case of the 

 Cetacea there are many sorts of pinnipeds, especially phocids or true 

 seals, which diflfer from one another in respects which for our present 

 purposes are of a rather minor character. The order is separable into 

 three well defined families, as discussed below. In addition there are 

 several curious fossil pinnipeds whose allocation may prove puzzling. 

 Because this order is of the utmost importance in a consideration of 

 aquatic modifications its characteristics will be discussed in considerable 

 detail. 



As an order the Pinnipedia are mammals of markedly stream-line 

 shape and with very short tails. TJie elbow and knee are always situa- 

 ted well within the body contour and the crotch is located at or slightly 

 above the level of the heel. The feet are webbed and in some are 

 paddle-like. The fifth toe of the hind foot is approximately as long 



[31] 



