All specimens examined of subspecies compared with the series of piñon mice from northeast of Durango are in the University of Kansas Museum of Natural History. Capitalized color names and designators are of Maerz and Paul, A Dictionary of Color, McGraw Hill Book Co., New York, 1930.
I wish to acknowledge generous financial aid from the Kansas University Endowment Association which made possible the field work by Mr. Alcorn in Mexico.
This heretofore unknown subspecies is characterized below and may be known as:
Peromyscus truei erasmus subsp. nov.
Type.—Mus. Nat. Hist., Univ. Kansas, no. 34417, young adult female, skin and skull; from eight miles northeast of Durango, 6200 feet, Durango, Mexico; collected 16 August 1949 by J. R. Alcorn, original number 10255.
Range.—Known only from the type locality.
Diagnosis.—Upper parts dark brownish gray (Smoke Brown, 16 A 2, to Biskra, 16 A 12), darkest between ears; lower sides suffused with dull orange buff (13 H 9 to 12 H 9); dark eye ring and black spot at base of vibrissae conspicuous; ears 95 to 100 per cent as long as hind foot; bullae round, greatly inflated; interparietal large, anterior margin curved or slightly sinuous, not bulging strongly forward laterally; rostrum short; nasals broad; braincase high and full; incisive foramina slightly pointed anteriorly; molars small, as in P. t. gentilis.
Measurements.—Measurements of 3 males and mean and extreme measurements of 11 females, all from the type locality, are, respectively, as follows: total length, 192, 188 (incomplete), 196 (incomplete), 193 (188-209); length of tail, 102, 97 (broken), 97 (broken), 101 (94-114); length of hind foot, 22, 23, 23, 22.5 (22-23); length of ear, from notch, in flesh, 21, 22, 23, 21.5 (20-23); greatest length of skull, 27.4, 27.7, 27.9, 27.3 (26.5-28.3); basilar length, 20.2, 21.0, — (broken), 20.4 (19.6-21.2); greatest breadth of braincase, 12.8, 12.8, 13.3, 12.85 (12.4-13.4); least interorbital breadth, 4.4, 4.6, 4.6, 4.41 (4.2-4.6); length of nasals, 10.1, 10.3, 10.9, 10.3 (9.8-11.1); diastema, 6.6, 7.0, 7.1, 6.78 (6.3-7.2); length of incisive foramina, 5.6, 5.9, 6.0, 5.77 (5.5-6.0); length of palatal bridge, 3.8, 3.9, —, 3.96 (3.8-4.3); postpalatal length, 9.9, —, —, 9.7 (9.2-10.4); alveolar length of maxillary tooth-row, 4.1, 4.1, 4.4, 4.2 (4.1-4.4). All measurements are in millimeters.
Measurements of the type.—Total length, 189; length of tail, 95; length of hind foot, 22; length of ear, from notch (in flesh), 21; greatest length of skull, 26.9; basilar length, 20.3; greatest breadth of braincase, 13.0; least interorbital breadth, 4.4; length of nasals, 10.1; diastema, 6.6; length of incisive foramina, 5.5; length of palatal bridge, 3.9; postpalatal length, 9.8; 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.