MURIDA. — CLXXXV. 319 
inhabiting the central plains; habits nocturnal and subterranean. 
Farther west occur numerous species of Saccomyide 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... . syle) GROM YS. 540); 
aa. Upper incisors not grooved; ears distinct but aos sant: claws moderate. 
THomomys, 541. 
540. GEOMYS Rafinesque. (yy, earth; pis, 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. TT. 3. Prairies, 
Wis., Ill., and W. to Rocky Mts. (Lat., pouched.) 
541. THOMOMYS Maximilian. (Owpos, heap; pis, 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. 23. Minn. to Utah and N. W. 
(Lat., like a mole.) 
Famiry CLXXXV. MURIDA. (Tue Mice.) 
Incisors 2; molars usually 3-3; 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 (Zus) 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, AZus. 
a. Incisors broad, often broader than deep; molars rootless (except in Evo- 
tomys) with flat crown and serrate margin; (body heavy, eyes small, 
snout blunt, legs short, ears small). (Arvicoline. ) 
6. Tail flattened, scant-haired; hind feet partly webbed. . . Friper, 542. 
bb. Tail subterete. 
c. Upper incisors grooved; ears large. - . » . SYNAPTOMYS, 543. 
cc. Upper incisors not grooved. 
d. Molars rootless; ears concealed; coronoid process of lower jaw reach- 
mg level of condyles s) Wye iga<) tes 44 tai? «, ARVICODA: 54d 
dd. Molars with roots; ears overtopping the fur; coronoid process of 
lower jaw not reaching levelof condyle. . . Evoromys, 545. 
aa. Incisors narrow, compressed; molars rooted, tuberculate, with crenate 
margin; (body slender; eyes and ears large; snout pointed; motions 
rapid). (Murine.) 
e. Molars of upper jaw with tubercles in two series ; palate ending opposite 
last molar. (American species.) 
