The two heretofore undescribed subspecies are characterized below and may be known as:
Baiomys musculus handleyi, new subspecies
Type.—Adult female, USNM No. 275604 (Biological Surveys Collection), skin and skull; from Sacapulas, El Quiché, Guatemala; obtained on April 24, 1947, by Charles O. Handley, Jr., original number 991.
Distribution.—Known only from the type locality; probably inhabits parts of the east-west drainage of the Río Negro.
Diagnosis.—General ground color of upper parts between Wood Brown and Buffy Brown (all capitalized color terms are those of Ridgway, Color Standards and Color Nomenclature, Washington, D. C., 1912), dorsal parts of fore- and hind-feet, and ankles white; in region of median venter, throat, and chin, hairs white to base; in lateral regions hairs Neutral Gray at base; dorsal hairs below tips Avellaneous, Neutral Gray at base; guard hairs black-tipped; tail white below, brownish above; nasals truncate anteriorly; frontalparietal suture forming an obtuse angle with median-parietal suture; alveolar-length of upper molar tooth-row and tail long.
Comparisons.—From Baiomys musculus nigrescens (paratypes, from the Valley of Comitán, Chiapas, México), found to the north, B. m. handleyi differs in: color paler dorsally and ventrally; fore- and hind-feet whitish instead of dusky to sooty; hairs in region of facial vibrissae white instead of brown; tail bicolored instead of unicolored; anterior tips of nasals square, not rounded; frontoparietal suture forming obtuse angle with median parietal suture instead of a right angle; tail and alveolar length of 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).