456 BULLETIN 56, UNITED STATES NATIONAL MUSEUM. 



orbitally, with a heavy supraorbital ridge extending to the occiput ; 

 occipito-nasal length, 36 mm.; zygomatic breadth, 20; tooth-row, 5. 

 This species may be instantly distinguished from texensis by its 

 yellowish instead of grayish belly, and its generally yellower colora- 

 tion and larger size. 



Genus REITHRODONTOMYS Giglioli (1873). 



Mus, Audubon and Bachman, Journ. Acad. Nat. Sci. Phila., VIII, 1842, p. 307. 



Hesperomys, Wagner, Wiegm. Arch., 1843 (2), p. 51. 



Reithrodon, Le Conte, Proc. Acad. Nat. Sci. Phila., 18.%, pp. 410, 413.— Baird, Mam. 



N. Am., 1857, p. 447 (not of Waterhouse). 

 Reithrodontomys Giglioli, Ricer. intorn. alia distrib. geog. goner., 1873, p. 60. 

 Ochetodon Coues, Proc. Acad. Nat. Sci. Phila., 1874, p. 1S4. 



Description. — Externally similar to Peromyscus. Size small. Tail 

 usually long and scantily coated with hair (fig. HF>c). Feet as 

 shown in fig. 115a, b. The molar teeth (fig. 115 d to g) resemble 

 those of Peromyscus, but the upper incisors differ in being strongly 

 grooved (fig. 115ft, i). 



Seven forms of this genus inhabit the region contiguous to the 

 Mexican Boundary Line. Five of these occur commonly on the line; 

 one in Sonora, 150 miles south of the line, and one in the Chiricahua 

 Mountains, Arizona, about 25 miles north of it. These forms may be 

 considered as belonging to two groups, one of which contains three 

 long-tailed, reddish forms of the so-called mexicanus group (Reithro- 

 dontomys mexicanus, R. laceyi, and R.fulvescens), the other four more- 

 grayish, short-tailed forms of the megalotis group (R. megalotis, R. 

 megalotis deserti, R. arizonensis, and R. longicauda). Dr. J. A. Allen, 

 in a recent paper on the genus Reithrodontomys , a considered the four 

 forms of the megalotis group as constituting three species (R. mega- 

 lotis, R. arizonensis, and R. longicauda) , and one subspecies (R. inega- 

 lotis deserti). I have followed him in this for the reasons that I have 

 seen no specimens of R. arizonensis, b or of intergrades between R. 

 megalotis deserti and R. longicauda, though 1 strongly suspect that the 

 form from southern California, recognized by Doctor Allen under the 

 name of Reithrodontomys longicauda pallidus I Rhoads), actually repre- 

 sents, in part, intergrades between R. longicauda and R. m. deserti, 

 the remaining specimens tabulated being true R. longicauda (espe- 

 cially those from the Coast Range Mountains, Jacumba, Nachoguero 

 Valley, Cameron's Ranch, San Isidro, Jamul Creek, Dulzura, and 

 Santa Isabel) and R. m. deserti (as those collected by Mr. Holzner, 

 at Seven Wells and Gardner's Laguna certainly are). In fact, while it 

 is certain that Mr. Rhoads described as Reithrodontomys pallidus a 

 specimen from Santa Isabel, California, which is indistinguishable 



a Bull. Am. Mus. Nat. Hist., VII, pp. 107-143, May 21, 1895. 



b It. i> possible thai this represents a connecting form between the mexicanus and mega- 

 lotis groups. 



