^38 PROCEEDINGS OF THE ACADEMY OF [1894. 



and curving strongly upward and inward; the coronoid notch nearly 

 vertical; the angle inflected (but not produced backward so far as 

 in Hodomi/s) ; the incisors slender, meeting in a single sharp point; 

 the molars very large; and m ;^, shaped like the letter S. Xoionuis 

 differs from Hodomij.-^ in having the mandil)le greatly reduced poste- 

 riorly; the condylar ramus longer and more slender, overt()p[)ing the 

 coronoid, and both coronoid and infracondylar notches larger at the 

 expense of the posterior part of the ramus, which is greatly reduced 

 thereby. In Xenomya the anterior base of the coronoid process 

 arises more anteriorly from the horizontal ramus, hiding the whole 

 of the last molar and posterior loop of the middle molar, while in 

 Hodoinys it arises further back, exposing the anterior loop of the 

 last molar. (In Ncotoma the line commonly falls between m 2 and 

 ;{ ; in Pti/ssophorus, according to Ameghino's figure, it apparently is 

 further back, exposing most of m 3.) 



Xenomys nelsoni Mcrriaiii. (PL IX, ligs. 10-13 ; and text tij;s. 3. f and /", and 

 fiS-4.) 

 Xenomys /ic/so/ii Mfrriani, Proc. Biol. Soc. Washington, VII, Sept. 1803, 161- 

 163. Typf from Hacienda Magdalena, Colinia, Mexieo. 



General chayacters. — Size about that of a half or two-thirds grown 

 rat, or nearly equaling Neotoma mexicana; tail a little shorter than 

 head and body, well haired, particularly above; face ornamented by 

 a distinct whitish spot over each eye and a less distinct one under 

 each ear; color of upper parts rich fulvous; under parts white; ears 

 about half as long as the head and nearly naked (sparsely clothed 

 with fine, inconspicuous hairs); whiskers reaching back to shoulders; 

 fur soft. 



Color. — Upper parts fulvous or tawny-rufous, palest on the head 

 and brightest over the rump, flanks, and hips; back sparsely mixed 

 with black-tipped hairs; an ill-defined dusky ring around each eye, 

 above which is a whitish spot about as large as the eye itself; a less 

 distinct whitish spot just below the inferior root of the ear; upper 

 lips white, the white color extending up on the cheeks more than 

 half-way to the eyes; sides of face below eyes and ears washed with 

 fulvous; whiskers blackish; tail concolor, dark undier-brown all 

 round ; up}jer surfaces of feet whitish , more or less clouded with 

 dusky (varying considerably in the three specimens); under parts 

 creamy white to the very roots of the hairs except along the sides 

 of the belly, where the basal part of the fur is plumbeous ; line of 



