478 RODENTIA 



Subfamily Bathyerginse. Angular part of the mandible arising 

 from the side of the socket of the lower incisor. Premolars absent 

 or present. Confined to the Ethiopian region. 



Bathyergus. 1 Upper incisors strongly grooved ; p \, m f ; no 

 ear-conchs ; very powerful claws. One species (B. maritimus), from 

 South Africa, attaining a length of about 10 inches. 



Geary chus* and Myoscalops. 3 Upper incisors without grooves. 

 Georychus, with some half dozen species, generally has p ^ ; Myo- 

 scalops, with one species, usually has p f , and the second toe of the 

 foot is the longest. In Georychus the premolar may be wanting, 

 and some examples of Myoscalops have only two teeth of this 

 series. 



Heterocephalus.* Small and nearly naked forms, with small 

 head, small eyes, no ear-conchs, moderately long tail, and powerful 

 fore feet provided with a pair of large pads; p $, m f^^L Two 

 species. These very remarkable little Rodents are regarded by 

 Mr. O. Thomas as very closely allied to Georychus, but specialised, 

 and, so to speak, somewhat degraded for a purely subterranean life, 

 for which their hairless body is peculiarly adapted. They are 

 found in Somali-land, where they burrow in the sandy soil. 



Family GEOMYID^:. 5 



Terrestrial or fossorial forms, with large cheek-pouches opening 

 on the cheeks outside the mouth. Squamosal much expanded, 

 and the jugal extending forwards to the lachrymal. P \ ; molars 

 rooted or rootless, with transverse laminae. Nearctic and Neo- 

 tropical regions. 



Subfamily Geomyinse. Incisors broad ; mastoid not appearing 

 on the top of the skull ; eyes small ; ear-conch rudimentary ; limbs 

 short, subequal. Habits fossorial. 



Geomys. Upper incisors deeply grooved. The common North 

 American Pouched-Rat or "Pocket-Gopher" (G. bursarius) inhabits 

 the plains of the Mississippi and lives in burrows. Several other 

 species are recognised from the Southern United States, Mexico, 

 and Central America. The genus is represented in the Pleistocene 

 and Pliocene of the United States. 



Thomomys. 7 Upper incisors plain. Represented by two species, 



1 Illiger, Prodromus Syst. Mamm. p. 86 (1811). 



2 Illiger, loe. tit. p. 87. 



3 0. Thomas, Proc. Zool Soc. 1890, p. ^8-Heliophobius ; Peters, Monatsber. 

 Ak. Berlin, 1846, p. 243. Preoccupied. 



4 Riippel, Mus. Senkenb. vol. i. Siiugeth. p. 99 (1834). 



5 Including the Saccomyidce of Coues. 



6 Eafinesque, Amer. Monthly Mag. vol. ii. p. 45 (1817). 



7 Wied, Nova Ada Ac. Cces. Leop.-Car. vol. xix. pt. i. p. 383 (1839). 



