HETEROMYIDJE. DIPODOMYINJE. DIPODOMYS. 341 



derive their trivial name. They possess cheek pouches, and these, 

 like those of the members of the GEOMYIDJE, are external, causing 

 these families to differ from all other Mammals in this respect. 

 These pouches are covered with hair on the inside as well as on the 

 outside. The skull of these animals is peculiarly shaped and very 

 light, being not thicker than a sheet of paper. The Kangaroo Rats 

 are divided into two genera, one with four toes on the hind feet, 

 the other with five; but in the latter the first digit, although possess- 

 ing a claw, is rudimentary. These handsome little creatures live 

 in burrows which they excavate in sandy soil, and they are preyed 

 upon by the spotted skunks, which are numerous in the localities 

 they frequent. They live in colonies and are very industrious, 

 digging their burrows, some of which are several inches in diameter, 

 in the shifting sand. They are hardy and do not seem to heed the 

 severest weather, and run about on the snow when the thermometer 

 registers below zero. At this time they feed on the seeds of cockle 

 and sand burrs, and in summer on the seeds of the prickly pear and 

 other hardy desert plants. They prefer the sandy districts near 

 rivers, where vegetation is scanty, and on sandy portions of the 

 prairies, and avoid those places where the soil is rich. They are 

 nocturnal, passing the day in sleep, and their tracks in the sand 

 each morning exhibit their activity and th.e extent of their night 

 wanderings. They are exceedingly pretty creatures, with a pleasing 

 coloring and fur as soft as silk, and with large, soft, expressive eyes. 



Fam. V. Heteromyidw. Kangaroo Rats. Pocket Jlice. 



Incisors narrow; molars rootless; mastoids enormously developed, 

 appearing on top of the skull; hind feet long; digits four or five; 

 pelage soft. 



Subfam. IV. Dipodomyinse. 



Anterior molar without lobe to the prism. Skull two-thirds as 

 wide as long, occipital plane emarginate; zygomatic plate of maxillary 

 nearly roofing the orbit; pit on inner side of jaw near molars. 



71. Dipodomys. Four-toed Kangaroo Rats. 



Dipodomys Gray, Ann. Mag. Nat. Hist., vn, 1841, p. 521. Type 

 Dipodomys phillipsi Gray. 



Macrocolus Wagn., Archiv. fur Natiirg., 1846, i, p. 172. 



Skull light, depressed, smooth, thin, broad posteriorly, tapering 

 anteriorly; anterior outline emarginate; rostrum extending beyond 



