by the light colored
race, _P. t. gentilis_, known from outside the area of lava rocks.
Specimens of _erasmus_ from eight miles northeast of Durango are
all conspicuously darker than 11 specimens of _gentilis_ from five
miles north of Durango and four miles west of Durango which are
typical in color for _gentilis_. Although _erasmus_ more nearly resembles
in color _gratus_, in cranial characters and external measurements
it shows closer relationship to _gentilis_.

Alcorn reported (verbal communication) that the type series of
_erasmus_ was collected on the west side of the Rio de la Saucida in
hills covered with broken lava rocks, cactus, and spiny shrubs.
Some cottonwoods grow along the river, which is almost dry most
of the time. East of the river is a flat plain or valley of adobelike
soil a few miles wide beyond which extends a rough area of dark
lavas. The approximate extent of the lava plain is indicated on
World Aeronautical Chart, Lake Santiaguillo (521). The specimens
of _gentilis_ from five miles north of Durango and four miles west of
Durango were collected on slopes of adobe soil covered with grasses,
scattered junipers and low shrubs, this habitat being the lower eastern
edge of the juniper-wooded slopes that rise westward to the
Sierra Madre Occidental.

The available facts suggest that _P. t. erasmus_ has evolved from
_P. t. gentilis_ by natural selection for concealing coloration on the
dark lavas northeast of Durango, Mexico. _P. t. erasmus_ probably
reaches its western limit close to the type locality.

_Specimens examined._--Total 14, from the type locality.

_Museum of Natural History, University of Kansas, Lawrence._

_Transmitted January 21, 1952._

24-2794

End of the Project Gutenberg EBook of A New Pinon Mouse (Peromyscus truei)
from Durango, Mexico, by Robert B. Finley

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