The series of four specimens, in the U. S. Biological Surveys Collection, from Grapevine Ranch clearly are not referable to _T. b. perpes_. They do, however, agree with _T. b. mohavensis_ in all essential particulars except that in two of the four specimens the braincase is wider and the nasals are wider distally. This width is evidence of intergradation with _T. b. pascalis_. Seemingly, then, they are best referred to _Thomomys bottae mohavensis_.
When Goldman (Jour. Washington Acad. Sci., 26(3):112, March 15, 1936) described and named this pocket gopher from Arizona, he arranged it as a full species and stated that there is no evidence of intergradation with other named kinds. We have examined the holotype and three topotypes (202579-202582 BS) and compared them with specimens of other kinds of pocket gophers occurring in northern and central Arizona. The _muralis_ gopher is a depauperate form clearly belonging to the _bottae_ group. The characters which Goldman (_loc. cit._) set forth as distinguishing _muralis_ from other named kinds are readily apparent and, like Goldman, we see no evidence of intergradation. Nevertheless, the characters which serve to identify the race are, in a general way, those commonly found in populations of depauperate individuals of _Thomomys bottae_ and _T. talpoides_. The small size, delicate structure, well-inflated braincase, short premaxillary tongues, and strongly recurved upper incisors, often appear in populations existing in inhospitable areas of shallow, unstable soils. For this reason we feel that the relationships of this population are best shown by arranging _muralis_ as a subspecies of _Thomomys bottae_; the name should stand as _Thomomys bottae muralis_ Goldman.
As far as known, _T. b. muralis_ is completely isolated from other populations of pocket gophers by uninhabitable eroding cliffs. The animals have been found only on isolated terraces in the lower end of Prospect Valley (itself a lateral pocket) within the Grand Canyon of the Colorado River, Hualpai Indian Reservation, Arizona. Consequently it is unlikely that intergradation with other populations could exist at the present time.
In short, in arranging _muralis_ as a subspecies of _Thomomys bottae_, we are influenced, not by the demonstration of intergradation, but by the degree of morphological differentiation of the population and the probable reasons therefor.