_Thomomys bottae tivius_ is the race closest geographically to _convexus_. From it, _convexus_ can be readily distinguished by: Size larger; tail shorter; hind foot longer. Color: Markedly lighter throughout. Skull: Much heavier and more compact, weights of skulls of males and females of the two subspecies being 2.4 grs., 1.6; 1.6, 1.2, respectively; rostrum heavier, wider and more depressed; zygomatic arches shorter, and more massive; upper incisors shorter, wider and more recurved; molariform teeth larger. For comparisons with _Thomomys bottae lenis_, _contractus_, _sevieri_, _bonnevillei_, and _robustus_ see accounts of those forms. _Remarks._--_T. b. convexus_ is limited to the area around Clear Lake in Millard County. This lake is surrounded by areas of loose, shifting sand and flat areas of barren alkali. The lake is fed by springs which flow from lava outcroppings on its eastern side. As far as discernible, the only area populated by pocket gophers (1938) was that adjacent to the lake where vegetation had trapped the sand. The factor which limits the extension of range of this subspecies probably is plant food. Also, the soil is mechanically poor for burrowing, since it caves in easily and burrows were found only in the sand where salt grass (_Distichlis stricta_) had trapped and stabilized it. Burrows were found from the edge of the water back as far as this grass persisted. _Specimens examined._--Total, 17, from the type locality. =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 Zooelogy, 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 Canon 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.