AQUATIC MAMMALS 



most certainly older than the ursine line, and the phocid older than that 

 of the otters. The resemblances are incontrovertible and undoubtedly 

 of real phylogenetic significance, but probably have no greater value 

 than indicating that the primitive ancestors of both otariids and bears, 

 although entirely distinct, had certain characters in common, as now 

 have any number of very diverse carnivores; and similarly with the 

 prototypes of the phocids and otters. 



Be that as it may these two groups of pinnipeds are now fundamentally 

 very different and have been distinct for a very long geologic time. 

 Their true phylogenetic dissimilarities are further accentuated by the 

 differences developed by their diametrically opposed methods of aquatic 

 progression. The walrus seems certainly a branch of the otariid stock, 

 modified by its enormous tusks, its feeding upon inactive prey, and 

 its bulk. The way in which it swims both with its fore feet like the 

 otariid and its hind feet like the phocid is extremely interesting and 

 significant. 



RODENTIA 



No rodent is very greatly modified for an aquatic life, probably be- 

 cause they are either dependent to great degree upon the banks of the 

 streams which they prefer, or feed upon plants which are terrestrial or 

 of the shallow water. In addition many of them are small enough to 

 be eaten by large fish if they venture into deep water. Besides web- 

 bing or bristly fringing of the hind feet in the more specialized sorts, 

 these members may be enlarged, the tail may be flattened, the fur 

 dense and soft, and the dorsal outline of the skull in some sorts is 

 less convex than usual, especially in the rostral region. 



Octodontidae 



Aiyocasfor — the coypu of eastern South America. This large rodent 

 has beneath its outer coat a suit of fine fur that is not so dense as 

 that of the muskrat but still heavier than one would expect to find 

 in any terrestrial rodent of the tropics. The hind feet are quite large 

 but the webbing of the toes does not extend to the tips. The tail is 

 perfectly round and the external ears are rather small. The mammae 

 are located dorsad. 



Hydrochoeridae 



Hydrochoeriis — the capybara of eastern South America is the larg- 

 est living rodent, attaining a body length of about four feet. The 

 nostrils are situated rather high, the toes are webbed only at their 



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