oyd, original number 377.

_Geographic distribution._--Eastern Coahuila, central Nuevo
Leon, 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 Leon, 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