508 
U. S. P. R. R. EXP. AND SURVEYS-ZOOLOGY-GENERAL REPORT. 
There are three genera of this sub-family, all of them found in North America, which may 
he briefly characterized as follows : 
Arvicola .—Field mice. Size small; soles naked anteriorly. Tail moderately short, cylindrical, 
hairy. 
Fiber. —Muskrat. Size large; soles naked; tail long, and much developed in a vertical 
direction; nearly naked. 
Myodes. —Lemmings. Size of Arvicola; tail very short; soles hairy. 
