From _B. m. brunneus_, _B. m. nigrescens_ differs in: dorsum blackish-brown rather than reddish to ochraceous brown; face and ears brownish-black rather than brownish with tinges of ochraceous; vibrissae darker; forefeet and hind feet darker; venter with more grayish tones; dorsalmost part of zygomatic plate projects farther anteriorly; interparietal oval to diamond-shaped and narrower anteroposteriorly; zygomata narrower at anteriormost part; slightly smaller in most cranial and external measurements. From _B. m. infernatis_, _B. m. nigrescens_ differs in: dorsum darker; region of face and ears darker; venter buffy to gray rather than whitish-buff; vibrissae darker; forefeet and hind feet darker; tail darker above and below; incisive foramina shorter, more constricted laterally; cranium slightly smaller in most dimensions. _Remarks._--Hooper (1952a:93-94) reported specimens from the coastal strip of southern Chiapas as the most intensely pigmented, whereas, specimens from central and western Chiapas were distinctly paler. Crania of specimens from the coastal region of southern Chiapas were smaller than crania from the central highlands and mountains of Chiapas. My studies essentially corroborate the findings of Hooper. The gradation of color between the pale brown _pallidus_ to the north in Oaxaca, and the brownish-black _nigrescens_ to the south in Chiapas is extremely gradual. Specimens from the central and western parts of Chiapas (see Figure 10 for localities) are difficult to assign to either _pallidus_ or _nigrescens_. Equal justification exists for assignment to either subspecies. I have assigned the specimens to _nigrescens_ because they are geographically closer to the type locality of _nigrescens_. Specimens from Reforma, Oaxaca (assigned by Hooper, 1952a:93-94, to _nigrescens_), are nearly identical in size and color to paratypes of _pallidus_. I assign the Reforma specimens to _pallidus_. The darkest of all the specimens examined and assigned to _nigrescens_ are from 1 mi. NW San Salvador and 1 mi. S Los Planes, El Salvador. The variations in color in this subspecies closely correspond to degree of relative humidity; the palest samples are from areas of low relative humidity and the darkest are from areas of high relative humidity. In view of the present state of differentiation of specimens from the southern coastal areas of Chiapas and mountainous areas of El Salvador, it would seem that populations there might be incipient subspecies.