DISTRIBUTION OF FIELD MICE. 9 



will apply as well to other genera and subfamilies of mice, while the 

 term " meadow mice " is not broad enough to include all the species 

 of Microtus. 



SPECIES AND DISTRIBUTION. 



Field mice of the genus Microtus have stout bodies, blunt, rounded 

 muzzles, small eyes, and short ears — often completely concealed in 

 the fur. The tail is short and hairy; the soles of the feet are naked 

 or clothed with short hairs, and have five or six foot pads (plantar 

 tubercles). The incisors are broad and not grooved. 



The molar teeth in all members of the genus, like the incisors of 

 all rodents, grow continuously throughout the life of the animal 

 and do not develop roots. They are prismatic in form, and the 

 crowns show triangular dentinal spaces surrounded by lines of 

 harder enamel. These curious enamel patterns are of great impor- 

 tance in the classification of the animals, as they are but slightly 

 affected by age and wear and are remarkably constant for each 

 species. 



About 165 living species and subspecies of Microtus have been 

 recognized (1904), of which about 78 are North American^ Eight 

 extinct fossil species have been described, and several of the living 

 forms also have been found fossil. 



The genus Microtus is of wide distribution, covering practically 

 the greater part of the northen hemisphere outside the tropical 

 zone — America north of the Tropics, all of continental Europe, and 

 Asia, except its southern peninsulas. Great Britain has two species, 

 but Ireland and the principal Mediterranean islands have none. 

 The genus is not found in South America, Africa, Australia, or the 

 Australasian islands. While true mice (Mus) are found over a 

 large part of the range of Microtus, they do not extend so far north- 

 ward and occur much farther southward. Microtus finds its most 

 southerly extension in the Old World in Palestine and the southern 

 slope of the Himalaya Mountains; in America, in southern Mexico 

 and Guatemala. 



The range of a single species is often remarkably great. Thus the 

 typical form of the common meadow mouse of the United States 

 (M. pennsylvanicus) occurs over a large part of at least twenty-five 

 States, from Maine to the Dakotas and southward almost to 35° 

 north latitude. If the five subspecies (geographic races) are in- 

 cluded, this range is almost doubled in extent. Another species, 

 M. mordax, occurs in most of the high mountains from Colorado to 

 California and from Arizona to Alaska. The meadow vole of Great 



a For key to the^ North American species and descriptions the render may 

 consult N. A. Fauna No. 17, Revision of Americau Voles of the Genus Microtus, 

 by Vernon Bailey, 1900. 



10700— No. 31—4)7 2 



