MURID^E. — CLXXX V. 319 



inhabiting the central plains ; habits nocturnal and subterranean. 

 Farther west occur numerous species of Saccomyidce or Pocket- 

 Mice, smaller than the Gophers, and with thin and papery skulls. 



a. Upper incisors, each with a large groove near the middle ; ears rudi- 

 mentary; fore claws enormous Geomys, 540. 



aa. Upper incisors not grooved; ears distinct but very small ; claws moderate. 



Thomomys, 541. 



540. GEOMYS Rafinesque. (yij, earth ; pvs, mouse.) 



1037. G. bursarius Shaw.) Pocket Gopher. Reddish brown, 

 with plumbeous tinge ; upper incisors with two grooves, the larger 

 near the middle line ; tail and feet hairy. L. 11. T. 3. Prairies, 

 Wis., 111., and \V. to Rocky Mts. (Lat., pouched.) 



541. THOMOMYS Maximilian. (0o>/x6j, heap ; (ids, mouse.) 



1038. T. talpoides (Richardson). Northern Pocket Go- 

 pher. Dusky plumbeous ; tail, feet and breast mostly white ; 

 ears in a dusky area. L. 9£. T. 2\. Minn, to Utah and X. \Y r . 

 (Lat., like a mole.) 



Family CLXXXV. MURIDiE. (The Mice.) 



Incisors f ; molars usually |:f ; anteorbital foramen a vertical 

 slit, widening above and bounded externally by a broad plate of 

 the upper maxillary ; coronoid and condyloid processes of lower 

 jaw well developed. Tibia and fibula united below. Genera 35 ; 

 species 300. A large family, found in all parts of the globe, some 

 of the species (Mus) being cosmopolitan, having accompanied man 

 in all his migrations ; all are of small size, the muskrat being one 

 of the largest, and some are smaller than any other quadrupeds, 

 except the Shrews. About one-third of the species belong to the 

 the Old World genus, Mus. 



a. Incisors hroad, often broader than deep : molars rootless (except in Evo- 

 tomys) with Hat crown and senate margin; (body heavy, eyes small, 

 snout blunt, legs short, ears small). (ArvicoliruB.) 

 b. Tail flattened, scant-haired; hind feet partly webbed. . . Fiber, 542. 



bb. Tail subterete. 

 c. Upper incisors grooved; ears large Synaftomys, 543. 



cc. Upper incisors not grooved. 

 d. Molars rootless ; ears concealed ; coronoid process of lower jaw reach- 

 ing level of condyle Akvicola, 544. 



dd. Molars with roots; ears overtopping the fur; coronoid process of 



lower jaw not reaching level of condyle. . . Evotomys, 545. 



aa. Incisors narrow, compressed ; molars rooted, tuberculate, with crenate 



margin; (body slender; eyes and ears large; snout pointed; motions 



rapid). (Murince.) 



e. Molars of upper jaw with tubercles in two series ; palate ending opposite 



last molar. (American species.) 



