Descriptions of East Asiatic Mammals 253 
strong digging claws. The cutting teeth are provided with very- 
hard enamel on their front faces and are used for gnawing 
through the trees, commonly aspen, which the Beavers use for 
building their remarkable dams and for food. 
The Beavers have special social instincts developed to a high 
degree. They seem better capable of communal planning and 
"forethought" than most other animals. The result invariably- 
aimed at by nature when she experiments with Beaver instinct 
is the raising of Beaver families in reasonable safety. First, 
the Beaver dam is made of mud and heavy sticks. This dam 
backs up the stream across which it is built, to form a deep pond 
which may cover several acres. In the pond the dome-like 
Beaver "lodges" are constructed of mud and wood, with their 
entrances under water. The young are born and raised on a 
platform in the lodge just above the level of the water. 
The Mongolian Beaver, Castor fiber birulai, looks to the 
uninitiated exactly like any Beaver in the United States. It is 
found rather locally in western Mongolia, and so in the strictest 
sense it is out of our territory. Beavers are reported also along 
the tributaries of the Kolyma River, which flows into the Arctic 
Ocean in eastern Siberia. A fossil Beaver, C. zdanskegi, has 
been found in Shansi, China. 
In prehistoric times there lived Beaver relatives, probably 
with unflattened tails, which burrowed as Woodchucks and 
Prairie-dogs do. Their burrows were spiral. Some of those 
burrows filled up with debris, which became fossilized into rock 
harder than the earth around it. Such spiral casts of burrows 
long remained without explanation and even received a scientific 
name, Daimonelix (devil's spiral)/ 
RAT-LIKE RODENTS ( SUBORDER MYOMORPHA) 
Just as the Squirrel suborder, the Sciuromorpha, contains 
such surprises as the Pocket Gophers and Beavers, so the 
Myomorpha includes, besides the Rats and Mice, such mammals 
