314 VERTEBRATE ANIMALS OF THE UNITED STATES 



Key to the United States Genera of Microtinae 



ai Tail round. 



bi Molars without roots (prongs) (Fig. 168). 



Ci Lower incisors long; their roots extending under the 

 posterior molars, 

 di Plantar tubercles mostly 6. 



ei Mammae mostly 8 i. Microtus. 



e2 Mammae 4 3. Pitymys. 



d2 Plantar tubercles 5. 



ei Mammas 8 2. Lagurus. 



62 Mammae 6 4- Neofiber. 



Co Lower incisors short, their roots ending on the inner side 



of the molars 5- Synaptomys 



hi Molars with roots (prongs) (Fig. 169). 



ci Back not red or chestnut in color; molars large 6. Phenacomys. 



C2 Back red in color; molars small and weak 7- Evotomys. 



0,2 Tail flattened laterally; size large; muskrats ■ 8. Fiber. 



I. Microtus Schrank. Meadow or field mice; voles. Cranium 

 slender; lower incisors very long, with roots ending on the outer side 



\ 



Fig. 168. — Lower jaw of Microtus pennsylvanicus, showing the molars without roots but 

 open below so that they grow continuously {from Miller). 



of the molars, which are rootless (Fig. 168); tail longer than the hind 

 foot; plantar tubercles 5 or 6; thumb with nail: over 150 species, ranging 



Fig. 169. — Lower faw of Phenacomys orophilns, showing the molars possessing roots (from. 



Miller). 



southward to the tropics; 30 species in the United States, mostly 

 northerly in range; both diurnal and nocturnal animals which build 



