_Remarks._--Blossom and Burt (1942:1) described _B. t. ater_ as the darkest of the known subspecies. It is dark, but specimens from some parts of the ranges of _B. t. analogous_, _B. t. fuliginatus_, and _B. t. subater_ exceed in melanins the darkest individuals of _ater_. Blair and Blossom (1948:5) also concluded by the use of an Ives tint photometer that _B. t. subater_ was significantly darker than _B. t. ater_. When paratypes of _ater_ and specimens of _B. t. paulus_ are compared, the darkest individuals of _ater_ exceed but slightly the darkest of _paulus_. The darkest specimens of _paulus_ occur in southern Zacatecas, and northern Jalisco, and the palest of the series are in northern Durango and southern Chihuahua. When paratypes of _ater_ and _paulus_ are compared, the difference in color is readily distinguishable. Specimens from 1-1/2 mi. N San Francisco, in northern Chihuahua, appear to be intermediate in color between _ater_ and _paulus_ except for a faint tinge of buff ventrally. In characters of the crania, these specimens resemble _ater_ and are referred to that subspecies. A slightly different pattern of color is present in pygmy mice from the Peloncillo Mountains and the Animas Valley of New Mexico; the upper parts resemble those of paratypes of _ater_, but the venter has only the faintest suggestion of the buffy wash. Crania of these specimens from New Mexico are inseparable from those of paratypes of _ater_, and the specimens are, therefore, referred to _ater_. When specimens are arranged by localities from Arizona east into southern New Mexico, thence south into Chihuahua and Durango, gradual intergradation in color is evident from dark in the north to pale browns in the south, whereas, size and shape of interparietal and size and shape of coronoid process of the lower jaw divide quite distinctly into two morphological types in central Chihuahua.