Cranial variation in size and proportion among adults is slight throughout the range of _ater_ compared to variation detected in other subspecies of _Baiomys taylori_. Perhaps such a relatively stable pattern of characters of the crania reflects the homogeneity of the gene pool, with respect to these characters, of the populations sampled. The fact that the color of the pelage of this subspecies varies considerable throughout its known range and that the crania do not is perhaps a clue to the mode of inheritance of characters in these mice. Seemingly, color of pelage is inherited independently of characters of the cranium. The relative lack of variability in the crania of _ater_ may result from uniform environmental conditions, which have served to select for uniform characters in the populations. All of the other wide-ranging subspecies of _B. taylori_ occupy more diverse habitats than _ater_. Secondly, the rather abrupt change in the cline of measured characters of the crania between _ater_ and _paulus_ in central Chihuahua suggests a secondary zone of intergradation. The probable cessation of gene flow in the past between these two subspecies, allowing _ater_ to be isolated for a time, may also, in part, account for the relative lack of variability in the crania of _ater_. _Specimens examined._--Total 58, distributed as follows: ARIZONA: _Graham County_: 1-1/2 mi. SW Ft. Grant, Graham Mts., 1[39]; _Pima County_: 1-1/2 mi. ENE Greaterville, Thurber Ranch, 2[39]; _Santa Cruz County_: Patagonia, 3[39]; _Cochise County_: _9 mi. W Hereford_, 10[43]; type locality, 2[43] (including the type); _5 mi. W Hereford_, 5[43]; 9-1/2 mi. W New Mexico State Line, 5-1/2 mi. N Mexican border, 20[42]; _3 mi. E, 1 mi. N Chiricahua_, 1[42]. NEW MEXICO: _Hidalgo County_: 18 mi. S, 2 mi. W Animas, 2; _22 mi. S, 2 mi. W Rodeo_, 6000 ft., 1[40]; _22 mi. S, 2 mi. E Rodeo_, 6000 ft., 3[40]; 25-1/2 mi. S Animas, 6200 ft. (in Big Bill Canyon), 1[40]. CHIHUAHUA: _5-1/2 mi. N, 2 mi. W San Francisco_, 5100 ft., 1; _2-1/2 mi. N, 3 mi. W San Francisco_, 5200 ft., 1; 1-1/2 mi. N San Francisco, 5100 ft., 4; Casas Grandes, 4300 ft., 1[41]. _Marginal records_--ARIZONA: 1-1/2 mi. SW Ft. Grant, Graham Mts. NEW MEXICO: 18 mi. S, 2 mi. W Animas; 25-1/2 mi. S Animas (in Big Bill Canyon). CHIHUAHUA: 1-1/2 mi. N San Francisco; Casas Grandes. ARIZONA: Patagonia; 1-1/2 mi. ENE Greaterville, Thurber Ranch. [39] University of Illinois, Museum of Natural History. [40] University of New Mexico. [41] U. S. Nat. Museum (Biol. Surv. Coll.). [42] University of Arizona. [43] Univ. Michigan, Museum of Zoology. =Baiomys taylori canutus=, new subspecies _Peromyscus taylori paulus_, Osgood, N. Amer. Fauna, 28:255, April 17, 1909 (part). _Peromyscus musculus_ [_musculus_], Osgood, N. Amer. Fauna, 28:256, April 17, 1909 (part).