328 VERTEBRATE ANIMALS OF THE UNITED STATES 



T.fuscus Merr. Color light brown; length 203 mm.; tail 70 mm.; 

 hind foot 27 mm.; mammae 4 pairs: Wyoming to Washington and 

 northward into Canada. 



Subspecies of T. fuscus 



T. f. fuscus Merr, Western Montana and Wyoming, northern and 

 central Idaho, eastern Oregon and Washington, British Columbia. 



T.f. saturatus Bailey. Size larger; color darker: higher parts of the 

 Coeur d'Alene Mountains. 



T. hesperus Merr. Color dark rich auburn; length 175 mm.; tail 54 

 mm.; hind foot 24 mm.: coast region of northwestern Oregon. 



Family 3. Heteromyidae. — Small, slender rodents with a long tail 

 and with a pair of cheek-pouches, which are lined with fur on the inside 

 and open at the side of the mouth on each cheek; eyes and ears usually 

 large; fur harsh in many species, with numerous bristles or spines: 

 6 genera and about 100 species, all in America, mostly in more or less 

 arid regions of the western States and Mexico. 



Key to the United States Genera of Heteromyidae 



ai Teeth 20 in number i. Perognathus. 



a2 Teeth 16 in number. 



bi Hind foot with 5 toes 2. Perodipus. 



1)2 Hind foot with 4 toes. 



Ci Tail with a terminal pencil 3. Dipodomys. 



C2 Tail without terminal pencil 4- Microdipodops 



I. Perognathus Wied. Pocket mice (Fig. 176). Body slender and 

 murine; tail at least as long or nearly as long as the rest of the body; 

 ears small; legs and feet long; dentition 1/1,0/0, 2/2, 2 /2 ; molars rooted 

 and tuberculate; upper incisors strongly sulcate; hair harsh in some 

 species and soft in others: many species, all in North America; 25 

 species and subspecies in the United States, all west of the Mississippi; 

 nocturnal, burrowing animals, dwelling on plains and deserts and feed- 

 ing on seeds; several litters of young raised annually, numbering from 

 2 to 8 each. 



Key to the Species of Perognathus 



ai Fur soft, without bristles; soles of hind feet more or less hair>^ (except P. 

 formosus). 

 bi Tail moderate in length and not crested. 



Ci Tail equal to or shorter than the head and body, 

 di In the Great Plains; length under 135 mm. 

 ei Tail about 60 mm. long. 



fi Color olivaceous P. fasciatus. 



