MAMMALS OF THE MEXICAN BOUNDARY. 



433 



cheeks, and rump ochraceous buff; top of head paler and grayer than 

 the back, which has no dark vertebral area; feet and under parts 

 pure white. In winter the coloring is darker than in summer. 



The young retain the gray coloring with which they are born until 

 they are two-thirds grown, when a coating of ochraceous and drab, 

 duller than that of adults, but otherwise much the same, is gradually 



Fig. 100.— Peromyscus ekemicus. Skull, a, dors.-vl view; 6, ventr.vl view; c, lateral view. 



acquired, appearing at first as a dull ochraceous band along the sides, 

 extending over the back at a later period. Five of these young desert 

 mice, in the mixed pelage of the young and adult, were described by 

 Doctor Merriam'^ as a distinct species — "Hesperomys (Vesperimus) 

 aniJionyi" — with the statement that three of them were adults, and 

 that "in coloration, proportions, and cranial characters this mouse 

 differs so radically from all previously known species that comparison 

 with others is unnecessary." Fortunately, the name anihonyi can 

 stand for the larger and darker subspecies of 

 Peromyscus eremicus inhabiting the Elevated 

 Central Tract between the eastern and western 

 deserts. 



Cranial and dental characters. — The skull (fig. 

 100) is low and flat, with a short rostrum and 

 truncate nasals ending posteriorly considerably 

 in front of the hinder extremities of the 

 premaxillaries. The brain case, though short 

 and flattened, is not so to the exaggerated degree 

 of P. auripectis. in which it is almost disk 

 shaped. The outer borders of the first and second upper molars have 

 but three salient and two reentrant loops or enamel folds (fig. 101). 



Habits and local distribution. — We found this handsome desert 

 mouse abundant from the Sonoyta River Valley to the western edge 

 of the Colorado desert, where, at the eastern slopes of the Coast Range 

 Mountains, it blends into the subspecies fraterculus. Those speci- 



FiG. 101.— Per OM Y s c u s 

 EREMICUS. Crowns of 

 molar teeth, a, lower 

 series; 6, upper series. 



aProc. Biol. Soc. Wash., IV, Apr. 15, 1887, pp. 1-3. 

 30639— No. 56—07 m 28 



