MAMMALS 305 



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. 



da Plantar tubercles 5. 



ei Mammae 8 2. Lagurus. 



ti Mammae 6 4. Xeotibcr. 



C2 Lower incisors short, their roots ending on the inner side 



of the molars 5- Synaplomys, 



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



Ci Back not red or chestnut in color; molars large 6. Pheiiacomys. 



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



a2 Tail flattened laterally; size large; muskrats 8. OiidiUra. 



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 very short but longer 

 than the hind foot; plantar tubercles 5 or 6; thumb with nail: over 



Fig. 169. — Lower jaw of Phenacomys orophilus, showing the molars possessing roots {jrom 



Miller). 



150 species, ranging northward to Arctic regions and southward to the 

 tropics; 30 species and very many subspecies in the United States, 



