From Peromyscus truei gratus (specimens from Pedregal de los Reyes, Distrito Federal, México) 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 Michoacán) 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 piñon 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 Río 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, México. 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.