432 



BULLETIN 56, UNITED STATES NATIONAL MUSEUM. 



This species, including its several subspecies, was found by us on 

 the Boundary Line from the Rio Grande at El Paso, Texas, to the 

 edge of the Pacific Ocean, and a related insular form was collectd on 

 Tiburon Island, in the Gulf of California, by Mr. J. W. Mitchell. The 

 species was originally described by Professor Baird from specimens 

 collected by Maj. G. 11. Thomas and Mr. Arthur Schott, a naturalist 

 of the earlier Mexican Boundary Survey, in the vicinity of Fort Yuma, 

 on the California side of the Colorado River. During the recent survey 

 of the Mexican Line we obtained a good series of 

 topotypes of Peromyscus eremicus from Fort 

 Yuma, San Diego County, California, from Yuma, 

 Arizona, and from the Colorado River bottom be- 

 low the mouth of the Gila River, on both sides of 

 the Colorado River. These were compared with 

 Professor Baird's types of eremicus (especially 

 No. "1334. Colorado Bottom, Cal. A. Schott"), 

 and found to be the same. Having definitely de- 

 termined the form of this species, which Baird 

 described as Hesperomys eremicus, it has been pos- 

 sible to describe other species and subspecies of 

 the desert-mice group, and to assign geographic 

 ranges to the subspecies of P. eremicus inhabiting 

 the Boundary Line strip. 



It would be difficult to frame a better descrip- 

 tion of the external characters of this beautiful 

 species than Baird gave in his original account 

 of it, but he neglected to say anything respecting 

 its skull and teeth, though the cranial and dental 

 characters of the group of desert mice typified by 

 Peroymscus eremicus are very characteristic. Mr. 

 Gerrit S. Miller, jr., in his description of the sub- 

 species /ra/^rfuZws", directed attention to the pecu- 

 liar form of the nasals and their relations to the 

 premaxillary and frontal bones, together with the 

 peculiar shape of the brain case. Still later the 

 writer described^ characteristic differences in the 

 tuberculation and in the enamel pattern of the 

 grinding surface of the middle and anterior molar teeth. 



Description. — In the typical form (subspecies eremicus) the soles of 

 the hind feet (fig. 99a) are naked to the heel; tail (fig. 996) longer 

 than head and body, not distinctly bicolor, slender, scantily haired, 

 and tapering to a fine point; ears large, nearly naked, thin, mem- 

 branous, and exceedingly sensitive. Color above grayish drab, tinged 

 with ochraceous, and not much darkened by black-tipped hairs ; sides, 



Fig. 99.— P ebomyscus 



EREMICUS. a, HIND foot; 

 b, TAIL. 



a American Naturalist, XXVI, p. 261. 



b Proc. U, S, Nat. Mus., XLX, 1896, p. 138 (p. 2 of advance sheet issued May 25, 1896). 



