Descriptions of East Asiatic Mammals 239 
of nipples (Blanford). The length of head and body is 23 inches, 
of the tail 6 inches, hind foot 3% inches, or more than half 
the length of the tail. 
This Marmot lives at very high altitudes, 14,000 to 15,000 
feet. It is an element of the Tibetan fauna that just reaches 
Szechwan and Yunnan. 
THE FLYING SQUIRRELS (FAMILY PTEROMYID^) 
The Flying Squirrels, known in America only by the single 
genus Glaucomys with its two species volans and sabrinus, are 
greatly developed in southern Asia, their present headquarters. 
Only the genus Pteromys (= Scinropterus) reaches Europe, 
and True Flying Squirrels are unknown in Africa, 
Flying Squirrels, though they have squirrel-like heads, bodies, 
and tails, possess also lateral fur-clad membranes that stretch 
from the front to the hind limbs and in some genera to the 
neck and tail also. At the outside of the wrist, the membrane 
is carried on a stiff rod of cartilage that acts as a spreader. 
The membranes contain sheets of muscles that can be tensed or 
relaxed at the will of the animals. Most of the genera are 
tropical. Some can scarcely be distinguished from one another 
except by minute characters of the skull. All species, so far as 
known, are nocturnal, and are reputed to sleep by day in holes 
in trees. They feed upon fruit, nuts, fresh bark, and grubs. 
The large species are said to be able to glide through the air for 
60 yards or so. Little is known about the habits of individual 
species. 
Probably the most convenient way to distinguish the Flying 
Squirrels from one another is by size. There are four large- 
sized genera, in which the length of the head and body equals 
or exceeds 10 inches. These are Petaurista (southeast Asia), 
Trogopterus (China), Aeromys (Malay Peninsula), and 
Aeretes (northern China). 
The remaining seven eastern Asiatic genera have the head 
