MAMMALS THAT GNAW. 107 



are several species from the Oriental countries, Tibet, and Abyssinia, the 

 minute eyes are open ; there are small naked external ears, and the short 

 tail is partially covered with hair. The other 

 four genera, all of which are confined to 

 Africa south of the Sahara, differ from other 

 Myomorpha, and thereby resemble the under- 

 mentioned Hystricomorpha, in that the angle 

 of the lower jaw arises from the side of the 

 sheath of the incisor. Of these, the great 

 sand-mole (Bathyergus maritimus) of the 

 Cape, which attains a length of 10 inches, has 

 grooved upper incisors, a single pair of pre- 

 molar teeth in each jaw, no external ears, 

 and extremely powerful claws. In the allied Georychus and Myoscalops the 

 incisors are smooth; the members of the former genus usually have a single 

 pair of premolars, and the single species of the latter three pairs of these 

 teeth in both jaws, while the second toe of the hind-foot is the longest. In 

 both, the first pair of premolars may be absent. The curious little naked 

 sand-rats (Heterocephalus), of which the two species are confined to Somali- 

 land, are degraded forms, with no premolars, either two or three- pairs of 

 molars, an almost completely bare skin, small eyes, no external ears, a tail 

 of moderate proportions, and a pair of large pads on the powerful fore- 

 feet. These tiny little animals make shallow tunnels in the hot sand of 

 the desert, throwing up at intervals small heaps resembling miniature vol- 

 canic craters. 



This comparatively small family is exclusively confined to North and 

 Central America, where it ranges from the plains of the Saskatchewan, in 

 Canada, southwards to Costa Rica, although attaining its 

 maximum development in the Western United States and The Pocket- 

 Mexico, and being unknown in the region east of the Gophers. 

 Mississippi, save the Gulf States. The essential characteris- Family 



tic of these Rodents is the possession of large pouches open- Geomyidce. 

 ing externally on the cheeks at the sides of the mouth. In 

 addition to three pairs of molars, they have a single pair of premolar teeth in 

 each jaw; all the cheek teeth in the more typical forms being rootless, and con- 

 sisting of simple prisms, without any unfolding of the enamel. The pre- 

 molars consist of a double prism, but all the molars, with the exception of 

 the last pair in the upper jaw of some species, comprise but a single prism, 

 whose summit presents an oval disc of ivory bordered by a ring of enamel. 



"All the members of the family," writes Dr. Merriam, " spend their entire 

 lives underground, and their whole organisation is modified in accordance 

 with the needs of a subterranean existence. The species, though numerous, 

 are very much alike externally They are short-legged, thick-set animals, 

 without any appreciable neck, without noticeable external ears, and with 

 very small eyes. The feet are largely developed for digging. The fore-paws 

 in particular are very strong, and armed with long curved claws, and the 

 sides of the toes are lined with rows of bristles that evidently serve in pre- 

 venting the dirt from passing between the fingers, thus completing a more 

 effective arrangement for keeping the tunnels clean, and for pushing the 

 earth out of the openings of the burrows. The tail, which is of moderate 

 length, is thick, fleshy, and usually devoid of hair, and is endowed with 

 tactile sensibility. 



