COLLECTION OF MAMMALIA. 321 



species, occupy the three following cases. The 

 most worthy of attention are the beavers, which 

 live in companies on the borders of rivers in Ca- 

 nada. With their teeth they cut down the trees, 

 which they use in the construction of dams, to 

 keep the water always the same height ; and they 

 build huts of two stories ; the low er apartment, 

 being under water, serves as a store-room, and the 

 upper they inhabit during winter. The industry 

 of these animals appears the more extraordinary, 

 when we consider their outward form. Their 

 number decreases, as so many are killed for the 

 sake of their fur. The substance employed in 

 medicine, under the name of castoreum, lies in 

 two glands under the belly of this animal. After 

 the beavers, we find the dormouse, remarkable 

 for sleeping during winter; the hamster, so de- 

 structive to corn, which it buries in its hole, 

 and is sometimes more than seven feet deep. 

 The chinchilla, so valuable for its fur ; the alac- 

 taga, a species of gerboa, given by M. Gamba ; 

 and the gerboa of the Cape brought by M. Dela- 

 lande. Near them are twenty-three species of 

 squirrels ; amongst which we see the flying 

 squirrel (pteromys), the skin of whose flanks, 

 extending into a membrane between the fore 

 and hind legs, enables it to remain a short 

 time in the air, and to leap with facili ty from 



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