_Specimens examined._--Total 60 as follows: VERACRUZ: 1 km. E Jalancingo, 6500 ft., 2; _2 km. S Jalancingo_, 2; 6 km. SSE Altotonga, 8000 ft., 8; _1 km. W Las Vigas, 8500 ft._, 2; Las Vigas, 8500 ft., 13; _2 km. E Las Vigas, 8000 ft._, 5; _3 km. E Las Vigas, 8000 ft._, 8; _5 km. E Las Vigas_, 7 (TAM); _5 km. N Jalapa, 4500 ft._, 2; _Jalapa_, 1 (USNM); 10 km. SE Perote, N slope Cofre de Perote, 10,500 ft., 1 (TAM); Xometla Camp, Mt. Orizaba, 8500 ft., 2 (BM); _Sta. Barbara, Mt. Orizaba, 12,000 ft._, 1 (BM); Xuchil, 6 (CM). #Peromyscus boylii ambiguus# new subspecies _Type._--Male, adult, skin and skull, No. 33092, United States National Museum, from Monterrey, Nuevo León; obtained on February 17, 1891, by Wm. Lloyd, original number 377. _Geographic distribution._--Eastern Coahuila, central Nuevo León, and the Sierra San Carlos, Tamaulipas. _Diagnosis._--Size small for the species; tail averaging longer than head and body (90-114%); dorsal coloration ochraceous, slightly darker middorsally; cheeks and lateral line Capucine Orange; skull small; supraorbital border rounded; anterior palatine foramina short. _Comparisons._--_P. b. ambiguus_ differs from _P. b. levipes_ in smaller size, longer tail relative to length of head and body, smaller incisive foramina, brighter and paler color, and relatively broader interorbital region. From _P. b. beatae_, _P. b. ambiguus_ differs in being smaller in all parts measured and paler. _Remarks._--Osgood (1909:155) reported as _P. b. levipes_ 37 specimens from Monterrey and 18 from Cerro de la Silla, Nuevo León, but noted that they were "aberrant." I have examined those same specimens and can hardly decide to which species, _P. boylii_ or _P. pectoralis_, they belong. Everything considered I, as did Osgood, opine that the specimens are _P. boylii_. However, I do not rule out the possibility that in this area there is an unnamed species, because I find an unusually wide range of variation in such cranial characters as size of the bullae, width and form of the pterygoid fossa, and shape of the braincase. Extremes of these characters are not constantly associated except in one specimen (33124 USNM), which is the smallest of all the adults examined. It has small bullae, a short rostrum, widely spreading zygomatic arches anteriorly, and a narrow pterygoid fossa, but does not differ externally from the other specimens. Additional material from this area is needed in order to make out the systematic position of these mice.