A Key to the Genera of North American Mammals 



M. Upper incisors grooved. 



SYNAPTOMYS, 107-108 

 MM. Upper incisors not grooved. 



N. Tail so short as to be barely visible 



beyond the hair. 



O. Color mottled (white in winter) . 

 DICROSTONYX, 108 

 OO. Color tawny orange. 



LEMMUS, no 

 NN. Tail plainly visible, usually about 



one inch long. 



O. Molar teeth rooted (See p. no). 

 P. Color dark - brown ; teeth 

 heavy . .PHENACOMYS, no 

 PP. Color rusty; teeth lighter. 



EVOTOMYS, III-II2 



OO. Molars not rooted. 



MICROTUS, 112-120 



LL. Tail equal to two-thirds the head and body. 

 M. Fur coarse; many buff hairs among 

 the brown ones; molars rooted. 



SIGMODON, 128-129 



MM. Fur fine uniform, like that of the 

 Muskrat; molars not rooted. 



MICROTUS ALLENI, 120 



KK. Slender with longer legs and prominent 



ears and eyes; tail always more than 



half the head and body (except Ony- 



chomys) , generally much more. 



L. Tubercles on molar teeth in three rows 



(introduced species). .. .Mus, 138-145 



LL. Tubercles on the molars, if present, in 



two rows (native species). 

 M. Size large, rat-like; molars flat on top, 

 divided into triangles. 



NEOTOMA, 127-128 



MM. Size medium, rat-like; molars tuber- 

 culate ; strongly resembling a 

 young Common Rat. 



ORYZOMYS, 129-130 

 MMM. Size small ; mouse-like. 



N. Tail always more than half the head 



and body; often about equal. 

 O. Upper incisors grooved. 



REITHRODONTOMYS, 130-131 

 OO. Upper incisors not grooved. 



PEROMYSCUS, 136 



NN. Tail short, less than half the head 

 and body ... .ONYCHOMYS, 136 

 HHH. Form squirrel-like; teeth, i |, c $, p f , m f . 



I. An extensible fold of skin on the sides of the 



body for flying. . . .SCIUROPTERUS, 176-178 



II. No extensible skin for flying. 



J. Burrowing animals; tail not bushy. 



K. Tail very short, 1-3 to 1-4 head and body. 



CYNOMYS, 160 

 KK. Tail 1-3 to 1-2 head and body. 



SPERMOPHILUS, 161-162 



KKK. Tail 1-2 head and body; body promi- 

 nently striped TAMIAS, 162 



299 



