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.

Remarks.—Miller's (Proc. Colorado Mus. Nat. Hist., 9:42, December 14, 1930) mention of a specimen taken on November 9, 1930, "near the head of Beaver Creek in extreme northeastern Elbert County" refers to the specimen, No. 2426 Colo. Mus. Nat. Hist., which is labeled as "8 mi. N. E. Agate, Elbert Co., Colo." Specimens from Colorado Springs, in the collection of the late E. R. Warren, have not been examined but the fact that Cary, Warren 1942, and Bailey (see under synonymy above) each referred the specimens to clusius instead of to the darker fossor gives basis for tentatively referring the specimens to T. t. retrorsus.

Grateful acknowledgment is made to those persons in charge of the mammal collections of the Denver Museum of Natural History and the Biological Surveys collection of mammals in the United States National Museum for permission to examine and report upon the material listed below (see specimens examined). The study here reported upon was aided also by a contract between the Office of Naval Research, Department of the Navy, and the University of Kansas (NR 161-791). Essential comparative materials were obtained with assistance from the Kansas University Endowment Association.