_Description._--Adults in dense unworn pelage taken in February at Two Buttes Reservoir: size large for the species; tail approximately 76 per cent as long as head and body; hind feet of medium length. Pelage: moderately long, thick; tail covered with short hairs; longest vibrissae 80 mm. Color: sides near Raffia (11 E 6) (capitalized color names and designators are of Maerz and Paul, A Dictionary of Color, McGraw-Hill Book Co., New York, 1930) overlaid with black, the general effect being grayish buff (13 G 6); back darker, moderately to heavily overlaid with black; indistinct dark eye ring; underparts whitish, fur basally gray except patch of fur pure white to base almost always present on upper throat; dark line around mouth; tail bicolor, black above, whitish below; feet white to ankles. Skull: large for the species, strongly arched at base of rostrum; rostrum heavy; zygomatic arches widely spreading, heavy, squarish; braincase moderately ridged and angular; nasals wide anteriorly, lateral margins nearly parallel or converging evenly posteriorly, tapered abruptly at posterior ends which reach posteriorly beyond anterior plane of orbits; dorsal branches of premaxillae extending 0.5 to 1.2 mm. posterior to nasals; interorbital region moderately channeled, narrowly constricted near middle of frontal (instead of anteriorly); supraorbital ridges concave laterally, diverging more strongly posterior to interorbital constriction (frontal 8.7 to 9.5 mm. wide at posterior ends of supraorbital ridges); temporal ridges widely flaring on parietals; occipital ridges prominent; interparietal broadly rectangular between temporal ridges, usually short in median line of skull, posterior margin straight or with slight median posterior angle; incisive foramina tapered toward both ends, sometimes narrower anteriorly than posteriorly; anterior palatal spine usually forming a blade thickened on ventral edge, and right and left sides usually incompletely fused; nasal septum with a posterior notch separating vomer from maxillary; posterior margin of palate usually bearing single or double point, sometimes straight; interpterygoid fossa moderately wide, lateral margins concave; sphenopalatine vacuities large; auditory bullae of medium size; basioccipital with low median ridge or crest; upper incisors wide, yellow or yellow-orange; molars large, M1 wider than M2; maxillary tooth-rows long, nearly parallel; anterointernal fold of M1 deep, cutting more than half way across first enamel loop. [Illustration: FIGS. 1-6. Skulls of _Neotoma mexicana_. All x 1. FIGS. 1-2. _Neotoma mexicana scopulorum_, holotype. FIGS. 3-6. _Neotoma mexicana fallax_, 1-1/2 mi. NW Golden, 6200 ft., Jefferson County, Colorado, 8 June 1948, [Male] No. 29182 KU.]