Pocket Mice 
65 
of the mouth on either side of the head; eyes large, and ears 
moderately so; elongated hind limbs and tails; skull with the 
mastoids largely developed and appearing at the top of the 
cranium; zygomata slender; incisors narrow; molars rooted 
or rootless. 
This family is chiefly distributed over western North 
America, some genera reaching south of Panama. 
Subfamily HETEROMYINiE (Pocket Mice) 
Very small forms with rooted molars and hind limbs only 
moderately lengthened. 
A small family confined to the New World. 
Subfamily DIPODOMYIN^ (Kangaroo Rats) 
Larger forms with rootless molars; skull triangular in shape 
with the mastoid enormously inflated; hind limbs very long 
with four or five toes. 
Key of the Colorado Genera of Heteromyid.e 
A. Molars rooted and tuberculate; tail shorter, less than ^ total 
length. (Pocket Mice) Perognathus, p. 65. 
B. Molars rootless and without reentrant angles; tail elongated, 
more than -| total length. 
(Kangaroo Rats) Perodipus, p. 71. 
Genus PEROGNATHUS (Grk. pera, pouch + gnathus, jaw) 
Perognathus Maximilian, Nov. Act. Acad. Ccbs. Leap. Carol., 
xix., pt. I, p. 369 (1839). Type P. fasciatvis. 
Revision, Osgood, N . A. Fauna, No. 18 (1900). 
This genus contains the Pocket Mice; these are desert-loving ro- 
dents of medium or small size with large swollen heads owing to the 
great development of the externally opening cheek pouches; the 
ears are small, and the tail is of moderate length, in Colorado species 
not exceeding by much if any the length of the head and body; the 
hind limbs are elongated, but not nearly to so great an extent as in 
Perodipus. 
In coloration the upper parts are of varying shades of buff 
grizzled with black, the under parts white or nearly so ; there is but 
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