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 

 be 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. 



