FOSSIL RODENTIA 51 
species of rodents than of all the other mammals put together, 
and they are found in all sorts of places; some are terrestrial, 
others arboreal, others fossorial or subterranean, others am- 
phibious. They live chiefly on vegetable food, such as grasses, 
fruit and nuts. 
During the Tertiary period Rodents were probably equally 
numerous; but their remains are so small that they are apt to 
escape the attention of collectors. Even so they are among the 
most common of fossil mammals. Most of them belong to 
families still living. 
Murip&. Rats, Mice, Muskrats, Meadow-mice etc. 
This is the largest group of the Rodents; there are more than 
170 recognized species in North America alone. Several species 
have been found rather abundant in the American Badland 
formations. In the White River beds, Eumys, allied to the 
White-footed Mouse, is common; Paciculus, allied to the Wood- 
Rat, is found in the John Day beds; Muskrats and Meadow-mice 
occur in the Pleistocene. 
GEOMYID2&. Pocket Gophers. 
Found only in North America. Fossil Pocket Gophers occur 
in the John Day and later formations in the Western States. 
HETEROMYID2. Pocket Mice. 
Found only in North America. Fossil Pocket Mice are 
rather common in the White River and John Day formations. 
SCIURID2. Squirrels, Prairie Dogs, Woodchucks etc. 
Fossil Squirrels, not easily distinguishable from the modern 
forms, are found in the Oligocene and later formations of the 
United States. Prairie-Dogs occur in the Pleistocene. 
HAPLODONTIID2, OR SEWELLELS. Oligocene to Recent. 
The Sewellel, or Mountain Beaver, is a peculiar little burrow- 
ing Rodent found only in the western Coast Region of North 
