7^ The Mammals of Colorado 
Thomomys Wied, Nova. Acta. Acad. Cess. Leop. Carol., xix., pt. 
I' P- 377 (1839). Type T. rufescens. 
Pocket Gophers which, though closely resembling the two follow- 
ing genera externally, are easily distinguished from them by their 
upper incisors which are smooth and without the median sulcus 
or groove, though there is a very fine one along the inner side of 
the tooth; enamel plates completely surround the premolar and 
molars of the upper jaw; the external edges of the crowns of the 
upper molars and the internal edges of the lower molars are drawn 
out into somewhat acute angles. 
This genus contains some 50 described species and has a 
more westerly and more extensive range than the two 
following, extending from about Lat. 55° in Alberta south- 
wards through the western half of the United States and the 
high plateau of Mexico to about the latitude of Mexico City. 
Examples of seven species and subspecies have been taken 
within the limits of the State of Colorado. 
Key to the Species of Thomomys 
A. Larger. Adults usually over 9 ins. total length. 
a. In fresh pelage, a bright red golden-brown color above. 
T. aureus, a, p. 79. 
b. Dusky brown above, blackish looking on back. 
T. aureus, bs p. 79. 
c. Color above chestnut, with rufous tinge, not golden, or else 
dull fulvous, with grayish tinge. T. a. pervagus, p. 79. 
B. Smaller. Adults usually under 9 ins. total length. 
a. Chin black. 
a'. Larger, length 8.75 ; cheek pouches white inside. 
T. fulvus, p. 80. 
b'. Smaller, length 7.8; cheek pouches dusky inside. 
T. c. ocius, p. 81. 
b. Chin white. 
a'. Whitish below, pale drab above. T. t. agrestis, p. 84. 
b'. Fulvous below. 
a''. Color dark, chestnut or chocolate, lower lips blackish. 
T. fossor, p. 82, 
b". Color paler, more of a buffy brown. T. clusius, p. 80. 
1 This applies to specimens from southwestern Montrose County ; 
some Montezuma County specimens also show this variation in 
color. 
