_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 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