Osgood suggested (1909:259) that the degree of relative humidity might in some way control color of pelage in this species. Relative humidity and its subsequent effect on other related environmental factors indeed may account for the superficial resemblance of _B. m. musculus_ to _B. m. handleyi_ (although _handleyi_ averages paler throughout than the paratypical series of _musculus_). Both subspecies inhabit relatively arid country. According to Goodwin (_op. cit._:39 and Plate 5, Fig. 1), and Handley (_in verbis_), the Río Negro Valley in the vicinity of Sacapulas is extremely hot, dry, and rather isolated. Extremes of climate there may exceed those in the arid habitat occupied by _B. m. musculus_. The resemblance between these two subspecies may result from nearly parallel selective forces that have given rise to two distinct subspecies. _B. m. handleyi_ may have developed _in situ_. _Specimens examined._--Total 49, from the type locality, including the type (12, USNM; 37, AMNH). =_Baiomys musculus pullus_=, new subspecies _Type._--Adult female, skin and skull, University of Kansas Museum of Natural History, No. 71605, from 8 mi. S Condega, Esteli, Nicaragua; obtained on July 15, 1956, by A. A. Alcorn, original No. 4218. _Distribution._--West-central Nicaragua, from Matagalpa northwest into the valley of the Río Esteli, east as far as Jinotega. _Diagnosis._--Dorsum Fuscus-Black (see remarks), individual dorsal hairs being black-tipped with a subterminal Ochraceous-Buff band, Neutral Gray at base; some hairs on dorsum all black to Neutral Gray at base; hair on sides Neutral Gray tinged with blackish; facial region blackish becoming more buffy ventrally; vibrissae black; tail unicolored Chaetura Black; fore- and hind-feet whitish to dusky-white; mid-ventral region of belly white to as far anteriorly as region of throat, hairs being white to base; in region of anus and throat, hairs white-tipped, Neutral Gray at base; tail long; upper molar tooth-row short as in _B. m. nigrescens_; zygoma bowed as in _B. m. grisescens_. _Comparisons._--From _B. m. grisescens_ (paratypes from Comayaguela, Honduras), _B. m. pullus_ differs in: dorsal ground-color and tail darker; sides and distal region of belly grayish instead of buffy-brown, thus making white stripe in region of belly distinct; average length of body and tail significantly longer, thus, total length greater; length of hind foot averaging longer (68 per cent confidence limits); alveolar length of upper molar tooth-row significantly shorter; occipitonasal and rostral length averaging longer; zygomatic spread and interorbital region narrower; length of incisive foramina, depth of cranium, postpalatal length, and breadth of braincase all averaging larger (see table 2).