262 Mammals of Eastern Asia 
following genera: the Red-backed Mice, Clethrionomys, with 
Aschizomys and Neoaschizomys; the Dawn Meadow Mice, 
Eothenomys; the Himalayan Mice, Alticola; the True Meadow 
Mice, Microtus \ the Rabbit-tailed Mice, Lagurus; the Mole-like 
Meadow Mice, Ellobius; and the peculiar Proedromys. These 
are all derived from common ancestry. The differences between 
them, not easily defined, depend chiefly upon tooth structure. 
But other characters aid in their distinction; the number of 
nipples in the females may assist in determination (8 in 
Microtus, 4 in Eothenomys) , also the length of the ears and 
tail and the form of the feet, including the number of pads and 
the types of claws. 
Clethrionomys, Lagurus, and Microtus are essentially north- 
ern genera; Eothenomys and Alticola, on the other hand, are 
centered around the Himalayan Mountains, many of them at 
great altitudes. Ellobius is a genus which reaches greatest 
development in central Asia. 
The Red-backed Mice, Clethrionomys, are widely distrib- 
uted in the cold temperate regions around the world. For the 
most part, they live in woods and often in cold, mossy or rocky 
forests. The general form is that of a Meadow Mouse. The 
color above is some shade of reddish brown, becoming paler on 
the sides, and gray beneath. The molar teeth have roots, instead 
of growing from persistent pulps. 
Of the four main divisions of this genus defined by Hinton, 
only the smaller-sized rutilus group and the larger-sized rufo- 
canus group occur in eastern Asia. The C. rutilus group (rutilus 
typifies the entire genus) contains, besides rutilus and its races 
from Europe to the Sea of Okhotsk, baikalensis and others 
from Lake Baikal eastward, irkutensis, laticeps, and parvidens 
from Irkutsk Province, C. r. jacutensis from Yakutsk, wosnes- 
senskii from Kamchatka, jochelsoni from the Kolyma River 
region in northeastern Siberia, amurensis from Amur and Sak- 
halin, and mikado from Hokkaido Island, Japan. The Mice 
