278 Merriam A New Rodent Genus from Mexico. 



not bullate as in Neotoma and Xenomys ; brain case depressed as in Pero- 

 myscus, not elevated as in Neotoma, Xenomys, and Hodomys ; incisive foram 

 ina large and open, broader anteriorly than in Neotoma or Peromyscus ; 

 coronoid process of mandible small, hardly larger than in Peromyscns. 



Dental characters (Fig. 14). Teeth rooted, large, massive, and prismatic, 

 with flat crowns presenting deep reentrant angles of enamel (enamel of 

 equal thickness throughout) as in Neotoma and Xenomys totally differ 

 ent from the small tubercular teeth (with enamel of unequal thickness) 

 of Peromyscus. Crowns of 1st and 2d upper and 1st lower molars with 

 enamel pattern essentially as in Neotoma, Hodomys, and Xenomys. Crowns 

 of 3d upper and 2d and 3d lower molars with enamel pattern unlike that 

 of any known genus ; 3d upper molar with a single very deep and narrow 

 reentrant angle on outer side, which pushes almost completely across 



the tooth , dividing the 

 crown into two sub- 

 triangular lobes, the 

 posterior of which is 

 the larger; 2d lower 

 molar with one re en- 



FIG. 14. Molar teeth of Nelsonia neotomodon and Neotoma trailt angle Oil each 

 desertorum. X 3- sidej the inner deeper 



a (upper) b (lower) molars of Neotoma desertorum. ^ and pas g in g an _ 



c (upper) d (lower) molars of Nelsonia neotomodon. 



tenor to, the outer, 



dividing the crown into two transverse loops the posterior of which is 

 sometimes deeply notched on the inner side by a secondary reentrant 

 angle ; 3d lower molar with a single and rather open reentrant angle on 

 inner side, reaching only half way across tooth, and a slight projection 

 (without distinct reentrant angle) on outer side. 



External characters. Size (of only known species) equalling largest 

 species of Peromyscus ; tail large and blunt as in Neotoma, and well haired, 

 with terminal pencil larger than usual in Peromyscus ; feet large, rela 

 tively as in Neotoma decidedly larger than in Peromyscus. 



Nelsonia neotomodon sp. nov. 



Type from mountains near Plateado, Zacatecas, Mexico. (Altitude 

 about 2500 meters or 8200 feet.) No. 90891, tf ad. U. S. Nat. Mus., 

 Biological Survey Coll. Collected Sept. 3, 1897, by E. W. Nelson and 

 E. A. Goldman. Original No. 11625. 



General characters. Size, coloration, and general appearance similar to 

 the larger species of Peromyscus (as P. calif ornicus) , but whiskers larger 

 and coarser, tail larger and blunter, and feet larger ; ears large and nearly 

 naked ; tail well haired, white- tipped, and with a distinct terminal pencil. 



Color. Upper parts grayish brown, conspicuously lined with black on 

 rump and posterior half of back, and suffused along the sides with pale 

 dull fulvous, which becomes intensified inferiorly so as to form a fairly 

 distinct band between the gray of the back and white of the belly, reach 

 ing all the way from cheeks to thighs; under parts white, the hairs 



