New Genus and Species of Murine Rodent. 161 



2 anterior and 1 posterior roots ; molar series large and heavy, 

 much broader than in Neotoma or Arvicola:, crowns flat, pris- 

 matic, non-tubercular, with broadly rounded and crowded alter- 

 nating closed triangles as in Phenacomys and Arvicola (only much 

 more crowded) and bearing no resemblance to the narrow trans- 

 versely elongated loops of Neotoma ; crown of last lower molar 

 deeply incised on the inner face a little anterior to the middle by 

 a narrow trenchant reentrant angle which carries a fold of 

 enamel obliquely forward and outward across the tooth. On 

 the outer side a short reentrant angle pushes a loop of enamel 

 obliquely forward and inward toward the middle of the long fold 

 from the opposite side, leaving an anterior loop, a posterior loop, 

 and a small closed or nearly closed triangle on the outside about 

 the middle of the tooth. The resulting pattern is shaped like 

 the letter S, with a small closed triangle on the outer side of the 

 convexity, thus differing widely both from the 3 transverse loops 

 of Arvicola and Phenacomys and from the 2 transverse loops or 

 rigure-8 pattern of Neotoma ; upper incisors plane or with a faint 

 bead on each side of the anterior face, which is nearly flat and 

 yellow ; lower incisors plane, convex in front, pointed. 



In describing genera from single species it is not always pos- 

 sible to distinguish clearly between generic and specific char- 

 acters, unless indeed the genus is based on a single character. 

 In the event of the discovery of additional species of the present 

 genus it may be found that some of the characters here ranked 

 as generic are only specific or, on the other hand, that some here 

 considered specific are really generic. 



The species may be known from the following description : 



Xenomys nelsoni ;; " sp. nov. 



Type No. i!i^ cf ad. U. S. National Museum (Department of Agri- 

 culture collection). From Hacienda Magdalena, Colima, Mexico, March 

 21, 1892. Collected by E. W. Nelson. (Original number, 2288.) 



Measurements (in millimeters, taken in flesh by collector). 

 Total length, 300; tail vertebra?, 143; hairs, 6;. hind foot, 30; 

 ear (in dry skin) from crown, 18 ; from anterior root, 22. 



General. Characters. Size about that of a half or two-thirds 

 grown rat, or nearly equaling Neotoma mexlcana; tail a little 

 shorter than head and body, well haired, particularly above ; 

 face ornamented by a distinct whitish spot over each eye and a 



"'Named in honor of its collector, Mr. E. TV. Nelson. 



