66 The Mammals of Colorado 
little seasonal change of color, the single annual moult taking place 
in late summer after the breeding season. 
In the skull the bullae are not as large as in Perodipiis, but do 
reach the upper side of the skull, while the zygomata are better de- 
veloped and have their roots free from the bullae. 
Dentition: i., \', pm. 3; m. | X 2 = 20; incisors grooved; molars 
rooted and tuberculate. 
This genus contains according to Elliot's Check-list 34 
species and 30 subspecies ranging over the western half of 
North America in the Sonoran and Transition Zones from 
British Columbia to the Valley of Mexico, and from the 
Great Plains at about Lon. 97° westward to the Pacific. Only 
six species have hitherto been met with in Colorado. 
Key to the Species 
A. Of smaller size, total length under 6.0 ; swollen portion of the 
mastoid bones projecting beyond the plane of the occi- 
put; interparietal bone very small, its width less than 
the interorbital space (subgenus Perognathus). 
a. Larger, length 5.0 or a trifle over. 
a'. Under parts buff. P. fasciatus infraluteus, p. 67. 
b'. Under parts white. 
a''. Color above pale buffy, with a rufous tinge, lightly 
lined with black. P. apache, a, p. 69. 
b". Ground color as above, but back very dark, blackish, 
sides clear rufous- tinged buff. P. apache, b.,i p. 69. 
c". Ground color above pale buffy, moderately lined with 
black, tail gray. P. flavescens, p. 68. 
d". Darker; grayish buff above, heavily lined with black; 
tail buffy. P. callistus, p. 68. 
b. Smaller; length 4^ or less. P. flavus, p. 70. 
B. Of large size, total length over 8.0; swollen portion of the 
mastoid bones not projecting beyond the plane of 
the occiput ; interparietal bone as wide as the f rontals 
at the interorbital space (subgenus ChcBtodipus) . 
P. hispidus paradoxus, p. 70. 
1 Specimens from southwestern Montrose County come under 
this head. 
