From topotypes of _Thomomys bottae trumbullensis_, _planirostris_ differs in: Size larger throughout; tail longer. Color: Much lighter throughout. Skull: More convex dorsally; rostrum wider and more depressed distally; extension of premaxillae posterior to nasals greater; zygomatic arches shorter, and not as widely spreading posteriorly; interpterygoid space more narrowly V-shaped; tympanic bullae smaller; upper incisors wider and longer; molariform teeth larger. Topotypes of _planirostris_ can be easily distinguished from those of _Thomomys bottae absonus_ by darker color throughout and markedly larger size. _Remarks._--From the synonomy at the beginning of this account one may note that the animals here ascribed to this subspecies have had nearly as many subspecific names applied to them as there have been investigators who have written about them. Although each of the previous writers had but a small amount of material upon which to base his opinion, the diversity of opinion as to subspecific status bespeaks the instability of these animals. The present study is based upon eighty animals including additional comparative material. All animals from Zion National Park have the characters pointed out by Burt (1931:38) in his description of this form. Farther down the Virgin River Valley towards St. George, however, some very perplexing problems of intergradation are encountered. St. George and environs may correctly be thought of as a "melting pot." Each of the fifty-seven animals studied from this region is an intergrade; some specimens combine the characters of three subspecies. As may be seen on the distribution map, three different subspecies of _Thomomys bottae_ occur in Washington County. Down the river, below St. George, the race _virgineus_ inhabits the Virgin River Valley below the narrows of the Beaverdam Mountains. Because these narrows are filled with water from wall to wall during periods of high runoff, they form an effective barrier at present to migration of pocket gophers. The mountains to the north of St. George are inhabited by the dark form, _birdseyei_. The type locality of _planirostris_ is on the middle reaches of the Virgin River, in Zion National Park. In addition Mount Trumbull to the south, in northern Arizona, is the locality of another subspecies, _trumbullensis_.