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). 



DlCROSTONYX, 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 



