A Key to the Genera of North American Mammals 



M. Upper incisors grooved. 



Synaptomys, 107-108 

 MM. Upp^r incisors not grooved. 



N. Tail so short as to be barely visible 

 beyond the hair. 

 O. Color mottled (white in winter) . 



DiCROSTONYX, 108 



00. Color tawny orange. 



Lemmus, 110 

 NN. Tail plainly visible, usually about 

 one inch long. 

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

 P. Color dark -brown ; teeth 

 heavy . .Phenacomys, iio 

 PP. Color rusty; teeth Ughter. 

 EVOTOMYS, 111-112 

 00. Molars not rooted. 



MiCROTUS, II2-I20 



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

 M. Fur coarse; many buf? 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, 130-13 1 

 00. Upper incisors not grooved. 



Peromyscus, 1^6 

 NN. Tail short, less than half the head 

 and body. . . .Onychomys, 136 

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



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

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

 IL 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 



899 



