New Genus and Species of 3[uri.nc Rodent. 161 



2 unteiioi- and 1 posterior roots ; molar series large and heavy, 

 much J)roader than in Xeotoma or Arvicola ; crowns flat, jiris- 

 niatic, non-tubercular, with broadh^ rounded and crowded alter- 

 nating closed triangles as in Phenaromj/s and Arrimla (only much 

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

 versely elongated loops of Neotoirui; crown of last lower molar 

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

 a narrow trenchant reentrant angle which caj'ries a fold of 

 enamel oblicpiely forward and outAvard across the tooth. On 

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

 (>l)li<piely forward and inward toward the middle of the long fold 

 from the oi)posite side, leaving an anterior loop, a i)osterior looj), 

 and a small closed or nearly closed triangle on the outside al^out 

 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 l)oth from the 3 transverse loops 

 of Arvicola and Phenacomys and from the 2 transverse loops or 

 figure-8 pattern of Neotoma ; ujjper incisors plane or with a faint 

 l_)ead on each side of the anterior face, which is nearly flat and 

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



In descril)ing genera from single species it is not always i)OS- 

 sible to distinguish clearly between generic and specific char- 

 acters, unless indeed the genus is based on a siitgle 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 tlie following description: 



Xenomys nelsoni ' yp. nov. 



Tyjje Xo. %lll\ c? ad. Y. S. Xutioual [Nliiseuni (Department of Agri- 

 culture collection). From Hacienda JNIagdalena, C'olima, Mexico, March 

 21, lcS92. Collected by E. W. Xolson. (Original numlxn-, 228S.) 



Measurements (in millimeters, taken in flesh l)y collector). — 

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

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



Generrd Characters. — Size al)out that of a half (jr two-thirds 

 grown rat, or nearly equaling Xeotoma ■nicxlcwKi, ; tail a little 

 shorter than head and l)ody, well haired, i)articularly above ; 

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



^•' Named in honor of its collector, Mr. E. W. Nelson. 



