7O VERTEBRATES : MAMMALS. 



2. Saccomyinae comprise those which have the body 

 graceful and slender and motions agile, the skull delicate, 

 muzzle long and tapering, tail very long, hind feet long, 

 and the fore claws moderate, but exceeding the hind ones. 



The Genus Dipodomys Kangaroo Rats is charac- 

 terized by a broad, depressed head, large, rounded ears, 

 acute snout, ample cheek-pouches opening externally, very 

 long hind legs, and a long tail with a brush-like tip. 



The Kangaroo Rat, D. Ordii, Woodh., of the eastern 

 slope of the Rocky Mountains, is about five inches long 

 to the tail, which is about as long as the body, with an 

 erect crest of long hairs towards the end. The color 

 above is yellowish-brown ; below, white. 



The Jumping Rat, D. Phillipii, Gray, and the D. agilis y 

 Gambel, belong to Western North America. 



The Genus Perognathus comprising the Tuft-tailed 

 Mouse, P. pencillatus, Woodh., and five or six closely 

 allied species, all of Western North America differs 

 from Dipodomys in having the ears small, tail of mod- 

 erate length, under surface of the soles naked or sparsely 

 hairy, the molars rooted and the transverse ridges tuber- 

 culated. The Tuft-tailed Mouse is three to four inches 

 long to the tail, which is four to five inches, with a 

 pencilled crest at its extremity. The color above is 

 yellowish-brown ; the under parts, hind feet, and fore 

 legs, white. 



MURID^E, OR RAT FAMILY. This Family comprises 

 Rats, Mice, and their immediate allies, in all more than 

 three hundred species, some of which are found in every 

 country on the globe. None are of large size, the musk- 

 rat being the largest, and some are the smallest quadru- 

 peds known, except the shrews. The dental formula is, 

 incisors | molars usually ~, rooted or rootless. Of this 

 large family, the most extensive of the whole order of Ro- 

 dents, there are three, at least, well-defined sub-families. 



