442 



BULLETIN 56, UNITED STATES NATIONAL MUSEUM. 



pure white below; under parts and feet pure white. The head is some- 

 what grayer, and there is a dusky orbital ring. 



Cranial and dental characters. — The skull is longer and narrower 

 in front than in the more western forms of P. eremicus, with the 

 rostrum somewhat longer. The outer border of the anterior and 

 middle upper molars have but three salient and two reentrant loops 

 or enamel folds, as is usual in the group of desert mice. The sub- 

 sidiary (often paired) marginal cusps, usually present in Perotnyscus, 

 are ol)solete in the eremicus group; and they are wholly wanting in 

 the genus OnycJiomys. Peromyscus eremicus affords another striking 

 illustration of the difference in the mammal forms of the Eastern and 

 Western Desert tracts. 



Ilahits and local distrihution. — The eastern desert mouse was found 

 in abundance about El Paso, where it lived among rocks, cacti, and 

 agaves, on both sides of the Rio Grande, specimens having been taken 

 from Texas and Chihuahua. Salt pork and oatmeal were attractive 

 bait. 



Measurements of 21 specimens of Peromyscus eremicus arenarius. 



