1 82 The Mammals of Colorado 
split up into a number of subgenera. In Elliot's Check-list 
38 species of North American squirrels are recognized, apart 
FIG. 59. PLAIN-BACKED SQUIRREL, SchlTUS aberti COUCOlOY 
Skull showing dentition x if 
from subspecies, but so far only two, including two sub- 
species, have been recorded from within our limits. 
Key of the Species 
A. Larger, head and body about 12 ins.; ears long and tufted 
about I inch; color above gray, with black or brown 
color phases. 
a. With a broad dorsal stripe of chestnut, S. aberti, p. 185, 
b. Paler, red dorsal area nearly obsolete, S. a. mimus, p. 183. 
c. Without the chestnut stripe, plain gray, S. a. ferreus, p. 184 
B. Smaller, head and body about 8 ins.; ears short, about 0.5 in., 
not tufted; color grayish on sides, rusty brown above. 
S. fremonti, p. 185 
