Genus Peromyscus 127 
Genus PEROMYSCUS (Grk. per a, pouch, + myscus, diminutive 
of mits, little pouched mouse). 
Peromyscus Gloger, Handb. und Hillsb. Naturg., i., p. 95 (1841). 
Type, P. leucopus noveboraccnsis. 
Revision, Osgood, North American Fauna, No. 28, 1909. 
Small mouse-like rodents with ears of varying size but often very 
large and rather hairless, nearly always projecting well beyond the 
fur; feet always white colored; tail long, bicolored, smooth, and 
moderately hairy, often tufted at the extremity and always more 
than half the length of the body; soles of the hind feet with the • 
usual six pads, the posterior part of the sole being slightly hairy; 
skull with the brain-case rather flattened; dentition, i. }; m. f X 2 
= 16; molars decreasing in size from in front backwards, when 
unworn with a distinct double line of tubercles; incisors smooth, 
not grooved. The young of this genus, in common with those of 
Onychomys, have a "blue" pelage, entirely distinct from that of the 
adults, and often persisting until the animal attains practically 
its full size. 
Of this genus containing the Deer -mice an enormous 
number of species have been described. Osgood's recent 
monograph of the genus gives 43 full species, and the grand 
total, including all the subspecies, is 142, which are spread 
all over North America from the Arctic Circle down into 
southern Mexico. Eight of these have been recorded from 
Colorado. 
Key of the Species 
A. Ears short and rounded; tail not pencilled, usually less than 
half the total length. 
a. Smaller. Total length usually less than 6.5. 
a'. Darker and brighter rufous. P. m. rufinus, p. 128 
b'. Paler and duller. P. m. nebrascensis, p. 131 
c'. Light tawny in color. P. m. luteus, p. 132 
b. Larger. Total length usually above 7. P. 1. tornillo, p. 133 
B. Ears large, but hardly equal to their distance from the tip of nose. 
Tail very long, more than half the total length, distinctly 
pencilled. 
a. Bright tawny yellow above, with patch of yellow on breast 
below. P. c. auripectus, p. 133 
b. Brown, mixed with blackish and white on back. No breast 
patches or spots. P. b. rowleyi, p. 134 
