upper molar tooth-row significantly larger (see table 1); zygomatic breadth, breadth of braincase, occipitonasal length, least interorbital constriction, and length of rostrum all averaging larger (see table 2).

From Baiomys musculus grisescens (paratypes, from Comayabuela, to the south, B. m. handleyi differs in: buff-colored hairs in dorsal and ventral region lacking; fore- and hind-feet white, not flesh-colored with gray overtones; tail bicolored, not unicolored; face paler, lacking buff-brown coloration; anterior tips of nasals squared rather than flaring; tail and upper molar tooth-row significantly longer (see Table 1); hind foot, ear from notch, and rostrum longer; braincase averaging broader (see Table 2).

Remarks.—The occurrence of these pale mice in the Río Negro Valley was first noted by Goodwin (1934:39, 40) when he referred specimens from Sacapulas and Chanquejelve, Guatemala, to B. m. musculus. Hooper (op. cit.:92-94) correctly assigned specimens from the southern coast and eastern part of Chiapas to B. m. nigrescens. The continued assignment of specimens from Sacapulas, Guatemala, to the subspecies musculus produces a hiatus both in the range of B. m. nigrescens and B. m. musculus. Twenty-four specimens, 14 from 1 mi. S Rabinal, and 10 from 1/2 mi. N, 1 mi. E Salama, Guatemala, are intergrades between handleyi and grisescens, but show more resemblance to the latter and, therefore, are referred to that subspecies. To the north, handleyi intergrades with nigrescens. The specimen from Chanquejelve is an intergrade between the two subspecies just mentioned.

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