76 



SKETCH OF THE HTSTORY OF MAMMALIA. 





I'amily. There are only two molars on each side above 

 and below ; the first above is three times the size of the 

 second, and is composed of three irregular portions, each 

 portion being depressed in its centre, which is sur- 

 rounded by a ridge of enamel ; the second molar is com- 

 posed of two unequal parts : the first molar below is 

 twice the size of the second, and both are composed of 

 two parts. (Fig. 44.) The Hydiomys is an aquatic 

 animal, and well adapted for swimming: the head is flat ; 

 the body otter-like, elongated, and covered with close 

 glossy fur : on the fore-feet there are four toes and the 

 rudiment of a thumb : on the hind-feet there are five 

 toes united by webs. The tail is long and cylindrical, 

 covered with close stiff hairs ; the ears are short and 

 rounded ; the upper surface is brown, the extremity of 

 the tail for about a third of its length white ; the under 

 surface varies; from white to a fine deep orange-yellow. 

 (Fig. 45.) Some writers have regarded these varieties 



