508 U. S. P. R. R. EXP, AND SURVEYS—ZOOLOGY—GENERAL KEPORT. 
There are three genera of this sub-family, all of them found in North America, which may 
be briefly characterized as follows: r 
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. 
