MAMMALIA. 463 



less of ri\'er3 or mountains ; and while no obstacle can impede their 

 progress, they devastate the country through which they pass. Their usual 

 residence appears to be the shores of the Arctic Ocean. Two species of 

 this genus are found in North America. 



The lemming of Hudson's Bay has become the type of the genus Afyodes, 

 which differs slightly from the genus Lemmus. Its specific name is M. hiid- 

 sonius. The two middle toes of the fore feet seem to have double claws, 

 which is owing to the skin at the end of the toe being callous and projecting 

 from under the nail. Of the size of the rat, and lives under the ground. 



The genus Fiber contains but one species, peculiar to North America, 

 the musk-rat {F. zihethicus). The lower incisors (two in number as above) 

 are sharp-pointed and convex in front ; the molars, three on each side, 

 above and below, have a flat crown furnished with scaly transverse zigzag 

 lamina3. The fore feet have four toes with the rudiment of a thumb, and 

 the hind feet five, the edge furnished with stiff hairs, which assist the 

 animal in swimming, the hind toes semi-palmated. The tail is long, com- 

 pressed, granular, nearly naked, having a few scattered hairs. A gland 

 near the origin of the tail secretes a white, musky, and somewhat offensive 

 fluid. The musk-rats are nocturnal in their habits, consequently their 

 manners and customs are difficult to observe. In winter they construct a 

 hut on the ice, in which several of them reside together. " A pond," say 

 Audubon and Bachman, " supplied chiefly, if not entirely, by springs, and 

 surrounded by low and marshy ground, is preferred by the musk-rats ; they 

 seem to be aware that the spring-water it contains probably will not be 

 solidly frozen, and there they prepare to pass the winter. Such a place, as 

 you may well imagine, cannot, Avithout great difficulty, be approached until 

 its boggy and treacherous foundation has been congealed by the hard frost, 

 and the water is frozen over ; before this time the musk-rats collect coarse 

 grasses and mud, with which, together with sticks, twigs, leaves, and any- 

 thing in the vicinity that will serve their purpose, they raise their little 

 houses from two to four feet above the water, the entrance being always 

 from below. We have frequently opened these nests, and found in the 

 centre a dry, comfortable bed of grass, sufficiently large to accommodate 

 several of them. When the ponds are frozen over, and a slight fall of snow 

 covers the ground, these edifices resemible small haycocks. There is another 

 peculiarity that, it appears to us, indicates a greater degree of intelligence 

 in the musk-rat than we are usually disposed to award to it. The animal 

 seems to know that the ice will cover the pond in winter, and that if it has 

 no places to which it can resort to breathe, it will be suffocated. Hence 

 you here and there see what are called breathing-places. These are covered 

 over with mud on the sides, with some loose grass in the centre, to preserve 

 them from being too easily frozen over. We have occasionally seen these 

 winter huts of the musk-rat, in the vicinity of their snug summer retreats 

 in some neighboring river's bank, and have sometimes been half inclined 

 to suppose that, for some cause or other, they gave a preference to this 

 kind of residence. We are not, however, aware that these nests are made 

 use of by the musk-rat in spring, for the purpose of rearing its young. We 



607 



