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.

Because of the wide range of variation in some of its characters, P. b. ambiguus is difficult to diagnose. Nevertheless, its small external and cranial size, short anterior palatine foramina, and bright color seem to separate it from other subspecies of P. boylii in the eastern part of the range of the species. These differences are most conspicuous when specimens from the northernmost part of the range of levipes are compared with specimens of ambiguus.

The specimens from the Sierra San Carlos, Tamaulipas, closely resemble levipes in color, but are referred to ambiguus on the basis of small size, as also are the two specimens from 12 km. E San Antonio de las Alazanas, Coahuila.