Thomomys bottae tivius Durrant
Thomomys bottae tivius Durrant, Bull. Univ. Utah, 28 (No. 4):5, August 18, 1937.
Type.—Female, adult, skin and skull, No. 1827, Museum of Zoölogy, University of Utah; Oak Creek Canyon, 6 mi. E Oak City, 6,000 ft., Millard County, Utah; September 14, 1936; collected by S. D. Durrant; original number 1100.
Range.—Limited to the Cañon Mountains, Millard County.
Diagnosis.—Size small (see measurements). Color: Upper parts Mummy Brown, grading through Cinnamon on the sides to Pale Cinnamon on the underparts; cheeks Cinnamon; postauricular patches black; distal third to half of tail white. Skull: Small, weak; zygomatic arches weak, not widely spreading, widest posteriorly; tympanic bullae large; interpterygoid space V-shaped; nasals short, usually simple distally, but with some denticulations in some specimens; palatal pits deep; palate narrow; paroccipital processes small; incisors, both upper and lower, narrow; molariform teeth small.
Comparisons.—Topotypes of tivius differ from those of Thomomys bottae albicaudatus as follows: Size markedly smaller in every measurement taken. Color: Lighter, Mummy Brown as opposed to near (13''''n) Black. Skull: Smaller, slenderer and weaker; zygomatic arches weak and not widely spreading as opposed to massive and wide spreading; nasals and rostrum narrower and shorter; extension of premaxillae posterior to nasals shorter; tympanic bullae smaller; molariform teeth smaller.
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 Cañon 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.