POCKET-MICE. 153 
the upper-parts is mouse-brown, becoming tawny on the flanks, while the under- 
parts, the tip of the tail, and a spot above each eye are white or yellowish. In the 
Rocky Mountains the place of this species is taken by Ord’s kangaroo-rat (D. ordi), 
which is a rather larger and more stoutly built animal, with a relatively shorter 
tail, and having five toes on each hind foot. 
The habits of the kangaroo-rats are very similar to those of the 
jerboas, these Rodents frequenting the most arid districts they can 
find, and living in burrows made beneath rocks or stones. In such districts there 
is no water, and but little vegetation save gigantic cactuses; and it appears that 
the food of the kangaroo-rats is formed by the roots, blades, and seeds of the 
seanty grass that manages to struggle into existence. Probably the only water 
that these creatures drink is that derived from dew collected on the cactuses. 
Little or nothing seems to be known of their breeding-habits. 
Habits. 
THE PocKkET-MICE. 
Genera Perognathus and Heteromys. 
The tiny little Rodents known in the United States as pocket-mice are dis- 
tinguished from the kangaroo-rats by the presence of roots to their molar teeth, 
and also by their inferior size—the whole length of the head and body seldom 
exceeding 2 inches. The genus Perognathus, as represented by the banded 
pocket-mouse (P. fasciatus), of North America, is characterised by the hair being 
coarse and bristly; whereas in the genus Heteromys, of which representatives 
extend as far southwards as Trinidad, the fur is mingled with a number of 
flattened spines. Most of these animals are brownish above and white beneath, 
with a tawny stripe on the flanks dividing the dark from the light area. 

