E. Raymond Hall

“ Examination (by Hall) of the specimens reveals that the differences relied upon by Burt and Hooper to differentiate the two species are well within the range of individual variation. For example, the variation (5.3 to 5.6 mm.) in width of the supraoccipital is less than in each of some other series of specimens of equal age of P. g. artus from other localities.
Also, there is geographic variation in the mice here assigned to the subspecies P. g. artus; skulls are smaller in the northern part of the geographic range and become gradually larger toward the south. ”

“ The uninflated mastoidal bullae "tip the balance" slightly in favor of artus to which the specimens from four miles north of Terrero are here referred. The degree of inflation of the mastoidal bullae elsewhere varies geographically. For example, the mastoidal bullae of the 25 specimens of goldmani from two and a half miles north of El Fuerte, Sinaloa, are intermediate in size as between those of goldmani and artus.
Map 1. Geographic distribution of Perognathus goldmani. The black dots are known localities of occurrence. ”