_Diagnosis._--Size medium (see measurements); upper parts ranging from between Cinnamon-Rufous and Hazel (capitalized terms are of Ridgway, Color Standards and Color Nomenclature, Washington, D. C., 1912) in the eastern part of the range to between Argus Brown and Brussels Brown in the western part of the range; sides Cinnamon-Rufous; throat whitish; remainder of under-parts whitish, in many specimens tipped with Ochraceous-Buff; feet and tail whitish; rostrum long; nasals ordinarily truncate posteriorly; temporal ridges nearly parallel; interpterygoid space broadly V-shaped. _Comparisons._--From _Thomomys talpoides clusius_ (topotypes), _T. t. rostralis_ differs in: Body longer; color more reddish (lighter with less brownish and more ochraceous); rostrum both longer and broader, actually and also in relation to length of the skull; skull broader interorbitally; upper molariform tooth-row longer; tympanic bullae less inflated. For comparison with _T. t. attenuatus_ to the east, see the account of that subspecies. From _Thomomys talpoides macrotis_ (topotypes) to the southeast, _T. t. rostralis_ differs in: Body shorter; upper parts slightly more ochraceous and less grayish; skull averaging smaller in all measurements except that interorbital region is broader and rostrum and upper molariform tooth-row are longer; nasals truncate versus emarginate, and consistently shorter; basilar length consistently less in specimens of equal age; mastoidal breadth less in 16 of 17 specimens of _rostralis_; temporal ridges parallel instead of divergent posteriorly; exposed parts of upper incisors shorter; tympanic bullae more angular antero-laterally. From _Thomomys talpoides fossor_ (specimens from Rico, Silverton, Hermit and Pagosa Springs, all in Colorado), the subspecies to the southward, _T. t. rostralis_ differs in: Longer body; lighter color of upper parts; nasals truncate rather than rounded posteriorly; temporal ridges more nearly parallel (less divergent posteriorly); rostrum longer (averaging longer and broader); skull wider across zygomatic arches in 11 of 12 specimens of _rostralis._