For comparisons with _Thomomys bottae stansburyi_ and _T. b. contractus_ see accounts of those forms. The four subspecies _tivius_, _albicaudatus_, _stansburyi_, and _contractus_ are the darkest in color of all the _Thomomys bottae_ occurring within the state. _Remarks._--This small, dark subspecies is limited to the Canon Mountains in eastern Millard County. Apparently it is a mountain derivative of _Thomomys bottae contractus_ which occurs in the valleys to the east and west of these mountains. Intergradation is noted with animals from the valleys on either side. For further comments on distributional problems of this type see remarks under _Thomomys bottae stansburyi_. _Specimens examined._--Total, 12, from the type locality. =Thomomys bottae contractus= new subspecies _Thomomys perpallidus albicaudatus_ Hall, Univ. California Publ. Zooel., 37:3, April 10, 1931. _Thomomys bottae albicaudatus_ Durrant. Bull. Univ. Utah, 28 (No. 4):4, August 18, 1937. _Type._--Male, adult, skin and skull, No. 1851, Museum of Zooelogy, University of Utah; Scipio, 5,315 ft., Millard County, Utah; September 17, 1936; collected by S. D. Durrant; original number 1125. _Range._--Extreme eastern Millard and Beaver counties, Utah. _Diagnosis._--Size medium (see measurements). Color: Upper parts Cinnamon Buff, mixed with black giving a color of Dresden Brown; sides between Cinnamon Buff and Pinkish Buff; underparts Pinkish Buff, purest on inguinal and pectoral regions; postauricular patches medium in size and black; ears covered with black hairs; nose, chin, cheeks and top of head dusky; front feet, hind feet and distal third to half of tail white; proximal part of tail covered all around with buff-colored hairs. Skull: Long, slender, moderately ridged and convex transversally at proximal ends of nasals; nasals long; rostrum long and narrow; posterior ends of nasals truncate or shallowly emarginate; ascending processes of premaxillae slender; extension of premaxillae posterior to nasals long; zygomatic arches neither robust nor widely spreading; interparietal subquadrangular; supraoccipital extending horizontally well behind lambdoidal suture instead of dropping off abruptly to the foramen magnum; interpterygoid space moderately V-shaped in some specimens, but somewhat lyre-shaped in others; tympanic bullae large and truncate anteriorly; upper incisors long and narrow; molariform teeth small and light.