AQUATIC MAMMALS 



If this be so, as seems probable, it is only to be expected that when 

 the tails of more primitive cetecea and sirenians had become sufficiently 

 specialized so that they were capable of sustaining higher speed than 

 the hind limbs alone were capable of attaining, the latter would be 

 folded back against the body and, playing a decreasingly important part 

 in the economy of the completely marine mammal, would finally suffer 

 atrophy and disappear. 



For the purpose of the present study the posterior limbs of aquatic 

 mammals will be discussed under several headings as follows: 



1. Hind feet the chief or exclusive swimming organs 



2. Hind feet used chiefly as equilibrators 



3. Hind feet absent or definitely subordinate in swimming 



In this simple classification there are naturally several forms placed under 

 one heading that might with almost equal propriety be put under an- 

 other. 



FIGURE 44. Dorsal outlines of the feet of a sea-lion (Zalophus}, seal (Phoca), 

 and elephant seal (Mirounga). 



1. Under this first heading are included (a) those forms in which 

 the fore feet are used but very little or not at all in swimming but which 

 have a tail of sufficient length to be of marked potential value in nata- 

 tion, although the latter is not as yet sufficiently specialized to be the 

 exclusive agent for propulsion; (b) all aquatic mammals which em- 

 ploy all four feet in swimming and whose tails are of insignificant size ; 

 and (c ) such forms as swim exclusively by means of the hind feet and 

 in which the tail is too small to be of use in this function. 



(a) Under this heading should be placed the single aquatic marsu- 

 pial Chironectes, all Insectivora except the insectivore otters Limnogale 

 and Potomogale, and all aquatic rodents except the capybara. 



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