_Thomomys talpoides clusius_, Bailey, N. Amer. Fauna, 39:100, November 15, 1915; F. W. Miller, Proc. Colorado Mus. Nat. Hist., 9:41, December 14, 1930; Warren, The Mammals of Colorado, Univ. Oklahoma Press, Norman, p. 162, 1942. _Type._--Male, subadult, skull and skin, No. 69840 Biological Surveys Collection, U. S. Nat. Hist.; from Flagler, Kit Carson County, Colorado; obtained on November 26, 1894, by Clark P. Streator; original No. 4460. _Range._--Western end of the Arkansas Divide in Colorado from eight miles south of Seibert westward to Colorado Springs _Diagnosis._--Size medium (see measurements); upper parts grayish brown; underparts lighter; skull small; tympanic bullae moderately inflated and angular anterolaterally; interpterygoid space narrowly U-shaped; pterygoid hamuli without transverse enlargement; nasals truncate posteriorly; premaxillary tongues projecting posteriorly behind nasals for distance of eight-tenths (0.5-1.1) of a millimeter. _Comparisons._--In comparison with _T. t. fossor_ and _T. t. rostralis_, which occur farther west, selected differences of _T. t. retrorsus_ are: lighter color; larger skull; more inflated tympanic bullae; greater relative (to length of skull) breadth across upper incisors, rostrum, and zygomata. The difference in color is greater in comparison with _fossor_ than with _rostralis_. In comparison with _T. t. macrotis_ (specimens from the type locality), _T. t. retrorsus_ is indistinguishable in color, length of tail, and length of tooth-row, but averages smaller in all other measurements. There is no overlap in length of body, basilar length, zygomatic breadth, mastoidal breadth or length of nasals. The temporal ridges, which mark the limits of the temporal muscles, are straight as opposed to curved and are lower. The tympanic bullae are more angular anterolaterally in _T. t. retrorsus_. From _T. t. attenuatus_ to the north, _T. t. retrorsus_ differs in darker (more brown) color, consistently longer body, relatively (to length of skull) shorter rostrum and nasals. Linear measurements of the two latter structures and length of tail are approximately the same in the two subspecies but all other measurements average more in _T. t. retrorsus_. Also in the latter the temporal lines are approximately parallel instead of being bowed outward in their middle extent and instead of being more widely separated posteriorly than anteriorly. From _T. t. cheyennensis_ to the northeast, _T. t. retrorsus_ differs in slightly darker (more brownish) color, consistently shorter body and rostrum, usually a more narrowly V-shaped interpterygoid space, and smaller average dimensions of the skull, notably in mastoidal breadth and length of the rostrum.