AQUATIC MAMMALS 



been led to believe that occasionally the tail is thus used alone, while 

 the feet are trailed. 



Neofiber the Florida water rat, is a smaller edition of the musk- 

 rat, but the tail is round, the feet are but slightly webbed, and hairy 

 fringes are lacking. 



Castoridae 



Castor the beaver, of North America and locally in northern 

 Europe. The under fur is famous for its softness and density. The 

 nostrils- are situated slightly higher than in most rodents, the external 

 ear is reduced in size, the hind feet are large and very fully webbed, 

 and the tail is naked, flat, and relatively broader than in any other 

 partially terrestrial mammal. Swimming is usually accomplished by al- 

 ternate movements of the hind feet, although occasionally I have seen 

 the tail alone used as a sculling oar. The tail is also used to facilitate 

 quick submergence, being then slapped loudly on the water as a warn- 

 ing signal: but it is not used as a trowel, as has frequently been 

 claimed in the past. 



Cricetidae 



There are three closely related genera of this family that are par- 

 tially aquatic. They occur in central and South America and have 

 short, dense fur. 



Ichthyomys. The muzzle is slightly flattened and the eyes and ears 

 unusually small. The upper incisors are prolonged into sharp lateral 

 points, which should facilitate the holding of slippery prey, and the 

 dorsal outline of the skull is definitely concave. The hind feet are 

 broad and fringed, but very incompletely webbed. The hairs on the 

 under side of the long tail are slightly lengthened and are dense, form- 

 ing a slight keel. 



Rheomys is very similar to the last genus but hairy fringes are 

 limited to the sides of the feet and outer toes only, and the tail is not 

 noticeably fringed. It is said to feed upon aquatic snails. 



Anatomy s is said to be similar to Ichthyomys also, but the ear open- 

 ing is reported to be a mere slit. 



As with the insectivores the line of aquatic modification which ro- 

 dents usually follow is in the webbing and fringing of the hind feet, 

 followed by enlargement of these members and a lateral flattening 

 or ventral fringing of the tail. External ears are said to be absent 

 in Anotomys but are present, although usually reduced, in all others. 



[37] 



