MAMMALIA. 323 
the month of May, attaining their growth in eighteen 
months, and living for nearly twenty years. They were 
formerly common in the New England States, where the 
remains of their dams can still be seen ; they are gradu- 
ally becoming extinct. Allied to the beaver is the curious 
Sewellel (Z7aglodon rufus) of the mountains of Oregon and 
Washington Territory. It is nocturnal, burrows in the 
ground, and is about the size of a muskrat. 
VALUE.—Twenty thousand beavers are taken yearly in Asia, and 
two hundred thousand in America. The incisors are used by the In- 
dians as chisels, knives, and ornaments. Beaver-leather is used, and 
castoreum in the manufacture of perfumery, 
Fic. 349.—The Myopotamus coypu, a valuable fur-bearing animal, at home 
in either salt or fresh water in South America. 
Rats (Jfuride).—In this family are the rats, mice, 
and their allies that are very generally distributed over 
are made of mud, and two-storied, the upper being out of water, in 
which the families live, while below are stored the provisions for the 
winter. The doors or openings connect with the water. The dams and 
home are repaired year after year, wood for the purpose being collected 
in the antumn, and when frozen the work is extremely solid. 
