8 AN ECONOMIC STUDY OF FIELD MICE. 
when food was exhausted, began to eat each other. They were finally 
destroyed by an infection- disease, which killed them by thousands.1 
In North America, up to the present time, no such calamitous 
invasions of field mice have been known, although occasionally one or 
another species increases locally to such an extent as to amount to 
a plague. Of even greater consequence, however, i- the steady drain 
on agricultural products caused by these rodents over a large part 
of the country when present in normal number-. 
CLASSIFICATION OF FIELD MICE. 
The genus Microtus belongs to a subfamily of Muridm known as 
the Microtince (formerly ArvicoMnce). This subfamily embraces a 
large number of forms of small and very similar rodents which in 
some respects resemble true mice, but which are readily distinguished 
by the robust body, thick head, short ears (whence Microtus), and 
short tail, the last exceeding two-thirds of the length of the body in 
but one genus. Fiber (muskrats). 
The Microtince are divided into two groups, which are commonly 
distinguished under the names lemmings and voles. Externally 
lemmings have shorter bodies than voles, and. except in one genus 
(Synaptomys) , the tail is shorter than the hind foot, and the palms 
and soles are without distinct foot pads (plantar tubercles). The 
two principal genera have the nails on the thumb strap-shaped 
(ligulate). 
Voles have bodies less stout, tails usually longer than the hind foot, 
and soles and palms always with distinct foot pads. The thumb 
nails are pointed, not ligulate. There are four genera of voles, all 
having representatives in the United States, of which the two more 
familiar are Fiber (the muskrats) and Microtus (the field mice). 
The other two genera are of little importance to the farmer. The 
present paper deals only with the genu- Microtus. 
Voles of the genus Microtus are distinguished from members of the 
other three genera by having rootless molars at all stages of their 
life. Fiber may easily be recognized by its large size, its long, later- 
ally flattened tail, and by its having the feet modified for swimming. 
There seems to be no entirely appropriate vernacular name for the 
mice of the genus Microtus. The French call them " campagnols,'' 
the Germans " wuhlmause." English-speaking people outside the 
United States call them " voles." In the United States they are 
variously designated as " meadow mice " or " field mice." and locally as 
" bear mice," ; * bull mice." " buck-tailed mice." " mole mice." etc. 
" Vole " is open to the objection that it applies equally well to three 
other genera and may easily be confused with " mole." " Field mice 1 
a Report on a Plague of Field Voles in Scotland. Appendix III. p. 76, 1803. 
