_Comparisons._--From _Thomomys talpoides bullatus_ (topotypes) to the northward, _T. t. attenuatus_ differs in smaller size, lighter (less brownish, more whitish) color, smaller and slenderer skull. In detail, some cranial features diagnostic of _attenuatus,_ when compared with _bullatus,_ are: Anterolateral angle of zygoma less nearly a right angle; temporal ridges bowed outward at middle, instead of straight, and farther apart posteriorly than anteriorly instead of nearly parallel; sides of basioccipital nearly straight instead of concave. From _Thomomys talpoides cheyennensis_ (holotype and Wyoming specimens from: Pine Bluff; 1 mi. W Pine Bluffs, 5000 ft.; 12 mi. N and 1/2 mi. W Pine Bluffs) to the eastward, _T. t. attenuatus_ differs in smaller size throughout and more slender skull. The two subspecies are indistinguishable in color. From _Thomomys talpoides macrotis_ (topotypes) to the southward, _T. t. attenuatus_ differs in smaller size, slightly lighter (less brownish and more whitish) color, smaller and slenderer skull. From _Thomomys talpoides rostralis_ (specimens from the type locality) to the westward, _T. t. attenuatus_ differs in smaller size; lighter (grayer, less brownish) color, smaller and less angular skull. From _Thomomys talpoides clusius_ (topotypes) to the northwestward, _T. t. attenuatus_ differs in shorter body, slightly grayer color, less width across mastoid region of skull, smaller tympanic bullae, and more obtuse anterolateral angle on zygoma. _Remarks._--This subspecies is of smaller size than any of the geographically adjoining subspecies. Intergradation with _T. t. cheyennensis_ is shown by specimens from two miles south and nine and one-half miles east of Cheyenne, Wyoming. Intergradation with _T. t. bullatus_ or _T. t. clusius_ or both is suggested by the larger size of the specimen from five miles southwest of Wheatland, Wyoming. Although large, this skull has the slender proportions of _attenuatus_ to which the specimen is tentatively referred. Although the specimens from Avalo, Colorado, are typical _attenuatus,_ the specimen from Pawnee Buttes, Colorado, is somewhat larger than typical _attenuatus_ and suggests intergradation with the subspecies to the southward, for example, at Flagler, Colorado. _Specimens examined._--Total number, 44, and unless otherwise indicated in the Museum of Natural History of the University of Kansas. =Wyoming.= _Niobrara County_: 10 mi. N Hatcreek Post Office, 5300 ft., 1. _Platte Co._: 5 mi. SW Wheatland, 1 (U. S. B. S.). _Goshen Co._: Little Bear Creek, 20 mi. SE Chugwater, 1 (U. S. B. S.). _Laramie Co._: 5 mi. W and 1 mi. N Horse Creek P. O., 7200 ft., 1; 3-1/2 mi. W Horse Creek P. O., 7000 ft., 6; 2-1/5 mi. W Horse Creek P. O., 6600 ft., 1; 2 mi. W Horse Creek P. O., 6600 ft., 2; Horse Creek 6500 ft., 1; 3 mi. E Horse Creek P. O., 6400 ft., 5; 6 mi. W Islay, 2 (U. S. B. S.); 2 mi. S and 1/2 mi. E Pine Bluffs, 5200 ft., 1; 7 mi. W