AQUATIC MAMMALS 



markably weak. It too belongs to a primitive family of insectivores 

 and its modifications probably began at a very remote time. 



CARNIVORA 



Ursidae 



Thalarctos the polar bear, is an inhabitant of the Arctic regions. It 

 spends so much of its time in the water that it must needs be included 

 in any consideration of aquatic mammals, yet the only way in which this 

 bear differs from others is in the increased hariness of the soles of the 

 feet, which was probably brought about by the coldness of the ice on 

 which it walks rather than any influence of the water. It also has a 

 build somewhat more rangy and slender in other words more stream- 

 line than is usual in this family. All four feet are employed in 

 swimming. 



Mustelidae 



Mustela. In this genus only the American mink and European sump- 

 fotter may be said to be aquatic, and yet there are really no discernible 

 external indications of the fact. The toes are partially webbed but so 

 are those of many strictly terrestrial species of this genus. 



Lutrinae. All river otters are very largely aquatic. Of this sub- 

 family the following genera are now recognized by those who have re- 

 cently worked with them: Microaonyx, Paraonyx, Aonyx, Amblonyx, 

 Hydrictis, Pteronura, Lutra, and Lutrogale. They occur almost through- 

 out the world on the larger and many of the smaller land masses. 

 The legs are rather short, the body sinuous and cylindrical, and the tail 

 is very stout, especially at base, tapering gradually from the body to the 

 tip. In Pteronura there is a lateral flange or keel upon each side of the 

 tail. The webbing of the feet differs somewhat in the different gen- 

 era, and this is discussed fully in the case of the Old World genera 

 by Pocock (1921) . Thus in Aonyx and Paraonyx of Africa the forefeet 

 are unwebbed, and even in the common otter they are no more webbed 

 than in many of the strictly terrestrial mustelids, although in at least 

 one species of this genus (Lutra macultcollis) they are. In Paraonyx, 

 Microaonynx and Aonyx, the so-called clawless otters of Africa, the 

 hind feet are practically unwebbed, but in most genera the webbing 

 extends almost or quite to the tips of the toes, although the feet are 

 little if any enlarged. The base of the tail is more robust than in ter- 

 restrial mustelids. 



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