434 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.47. 



ONYCHOMYS LEUCOGASTER (Wied). 

 (Synonymy under subspecies.) 



Diagnosis. — Size averaging considerably larger than in Onychomys 

 torridus, stouter and heavier built; tail relatively short, usually less 

 than half the length of head and body. Skull averaging larger than 

 in torridus, with comparatively narrow interorbital region. Teeth, 

 as compared with teeth of torridus, higher crowned; unworn cusps of 

 m^ higher than long; the anterior cusp more coniform, with less 

 indication of incipient division at summit into two ot three cusplets ; 

 m^ less narrow and elongated, stouter and relatively short; m' 

 larger than in torridus, longer than wide, or subcircular with longitu- 

 dinal and transvei*se diameters nearer equal; the crown surface usually 

 about one-third that of m^ (sometimes nearly one-half, rarely about 

 one-fourth) ; owing to the shortening of m ^ and the enlargement of 

 m^, m ^ is almost invariably less than half the length of tooth row. 



Geographic distrihution. — Western United States and Canada and 

 northern Mexico. From the Great Plams of Alberta, Saskatchewan, 

 and Manitoba south into Sonora, Chihuahua, and Tamaulipas, 

 Mexico. East to western Minnesota and eastern Kansas; west to 

 central Washington and Oregon, extreme northern and eastern Cali- 

 fornia, and, in Arizona, to the Grand Canyon. Absent from large 

 areas in the higher Rockies, the Pacific coast region, the Colorado 

 and Mohave Deserts, and the extreme southwestern United States in 

 general (fig. 2). 



Subspecies. — Twelve well marked geographical races of OnycJiomys 

 leucogaster are recognized. Most of these forms occupy large areas 

 which correspond well with the mapped zones and faunal regions of 

 the Western States. Two local variations from lava-bed and sand- 

 dune districts are imperfectly known. The most difl^erentiated forms 

 are from the northwestern and extreme southeastern parts of the 

 range. 



ONYCHOMYS LEUCOGASTER LEUCOGASTER (Wied). 



1841. Hypudaeus leucogaster Wied, Reise in das inuere Nord-America, vol 2, 



p. 99. 

 1857. Hesperomys leucogaster Bairi), Gen. Rep. North Amer. Mamm., p. xxviii. 

 1857. Onychomys leucogaster Baird, Gen. Rep. North Amer. Mamm., p. 459. 

 1857. Hesperomys (Onychomys) leucogaster Baird, Gen. Rep. North Amer. Mamm., 



p. 480. 

 1885. 0[nychomys] leucogaster var. pallidus Herrick, Geol. and Nat. Hist. Siirv. 



Minnesota, 13th Ann. Rep. (1884), p. 183. (Lake Traverse, near sources of 



the Minnesota and Bois des Sioux Rivers, South Dakota.) 

 1885. 0[nychomys] pallidus Herrick, Geol. and Nat. Hist. Surv. Minnesota, 



13th Ann. Rep. (1884), p. 184. 

 1888. Cricetus leucogaster Thomas, Proc. Zool. Soc. London, 1888, p. 133. 

 1888. C[alomys'\ leucogaster Jordan, Man. Vert. Anim. North. U. S., ed. 5, p. 321. 

 1912. Onychomys leucogaster leucogaster Miller, Bull. 79, U. S. Nat. Mus., p. 



127. December 31. 



