; alveolar length of maxillary tooth-row, 4.2.

_Comparisons._--From _Peromyscus truei gentilis_ (specimens from 5 mi. N
Durango, Durango; 4 mi. W Durango, Durango; and 8 mi. N & 1 mi. W
Sombrerete, Zacatecas), the subspecies of the surrounding region, _P. t.
erasmus_ differs in markedly darker coloration, sides and face less
brightly washed with orange buff, dark eye ring and spot at base of
vibrissae more conspicuous, higher incidence and greater extent of buffy
pectoral spot. External measurements do not differ significantly. No
consistent cranial differences were found.

From _Peromyscus truei gratus_ (specimens from Pedregal de los Reyes,
Distrito Federal, Mexico) to the southeast, _P. t. erasmus_ differs in
slightly darker dorsal color, more inflated bullae, and less sinuous (not
bulging so much forward laterally) anterior margin of interparietal.

From _Peromyscus truei gratus_ (specimens from various localities in
eastern Jalisco and western Michoacan) to the south, _P. t. erasmus_
differs in slightly darker dorsal color, longer ears, and more inflated
bullae.

From _Peromyscus truei truei_ (specimens from 4 mi. N El Rito, Rio Arriba
Co., New Mexico) to the northwest, _P. t. erasmus_ differs in much darker
color, shorter tail, shorter hairs on tail, smaller ears, shorter rostrum,
wider nasals, and more pointed anterior ends of incisive foramina.

_Remarks._--The tail of _P. t. erasmus_ varies greatly in color, being
either bicolor or unicolor, dark gray above and varying from white
to dark gray below. The type has the tail dark gray above grading
gradually on the sides to medium gray below. A buffy pectoral
spot or band is present in about half of the adults examined, being
most prominent in the type, which is also one of the darkest specimens
in the series. The shape of the posterior edge of the bony
palate is also variable, being convex, square, or concave; and the
dorsal branches of the premaxillaries may terminate slightly anterior
or slightly posterior to the posterior ends of the nasals. In the type
the posterior palatal margin is concave and the dorsal branches of
the premaxillaries almost reach the ends of the nasals. _Peromyscus
truei gratus_ from Distrito Federal also shows high variability in all
these characters.

_Peromyscus truei erasmus_ is a dark race of the pinon mouse known
from the west side of a rough area of dark lavas a few miles northeast
of the city of Durango and closely surrounded