Comparisons.—Compared with topotypes of Thomomys bottae osgoodi, aureus differs as follows: Size larger in every measurement taken, except tail which is shorter. Color: Darker throughout except on ventral surface which is lighter. Skull: Larger, longer and wider; nasals longer; rostrum wider and longer; zygomatic arches more nearly straight and heavier; ascending processes of premaxillae wider; basioccipital longer; interpterygoid space U-shaped as opposed to V-shaped; tympanic bullae larger; upper incisors longer, wider; molars larger.
Topotypical specimens of aureus can be distinguished from those of Thomomys bottae dissimilis by: Size larger throughout. Color: A trifle darker on dorsal surface. Skull: Larger in every measurement taken; zygomatic arches heavier and more nearly straight; tympanic bullae larger and more inflated ventrally; interpterygoid space U-shaped as opposed to V-shaped; alveolar length of upper molar series longer; molars larger; upper incisors longer and wider.
Topotypes of aureus differ from those of Thomomys bottae absonus as follows: Size larger in every measurement taken. Color: Darker dorsally, Light Ochraceous as opposed to Cinnamon Buff; due to admixture of gray, absonus has more of a grayish cast. Skull: Larger in every measurement taken, longer, narrower and more compact; zygomatic arches heavier; ascending processes of premaxillae wider; jugals heavier; tympanic bullae larger; interpterygoid space U-shaped rather than V-shaped; upper incisors longer and wider; molars larger.
From topotypes of Thomomys bottae planirostris, aureus can be distinguished as follows: Size larger; tail shorter. Color: Lighter throughout. Skull: Larger in every measurement taken except zygomatic breadth, extension of premaxillae posterior to nasals, and length of upper molariform series which are less; rostrum longer, wider and more convex; nasals slightly arched rather than straight; depression absent rather than present in posterior region of nasals; zygomatic arches not so widely spreading, but equally heavy.
For comparisons with Thomomys bottae alexandrae, see accounts under that form.
Remarks.—Topotypes of aureus are among the largest pocket gophers in the state. They are exceeded in total length only by T. b. lenis and are approached by T. b. aureiventris and T. b. planirostris. On the average they have the longest hind foot, body and ear. The length of the skull is second only to that of lenis as also is the length and breadth of the rostrum relative to the basilar length.
From the time of the original description of aureus in 1893 until 1930, all light colored gophers from Utah were referred to that form. Barnes (1927:100) gives the range of aureus as extending completely across southern Utah and on the west and east sides as far north as central Utah. Since 1930, forms named by E. R. Hall, W. H. Burt, E. A. Goldman and the writer have restricted the range of aureus in Utah to that part of the state east of the Colorado River.
Specimens examined.—Total, 22, as follows: San Juan County: Bluff, 3,300 ft., 22 (15, M. V. Z.).
Thomomys bottae birdseyei Goldman
Thomomys bottae birdseyei Goldman. Proc. Biol. Soc. Washington, 50:134, September 10, 1937.