_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.