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, I10 
NN. Tail plainly visible, usually about 
one inch long. 
O. Molar teeth rooted (See p. 110). 
P. Color dark-brown ; teeth 
heavy .. PHENACOMYS, I10 
PP. Color rusty; teeth lighter, 
EvotTomys, Ifi-112 
OO. Molars not rooted. 
MICROTUS, 112-120 
LL. Tailequal to two-thirds the head and body. 
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, 1 1, c 2, p 2, m 3 
I. An extensible fold of skin on the sides of the 
body for flying....ScruropTERuUS, 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 
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