128 Merriam New Murine Rodents from Mexico. 



transverse and the two ends of each crown more alike ; m^ a cylindrical 

 peg ; enamel pattern of lower molars in general like that of Hodomys, 

 with differences in detail : m T and m^ with 3 salient loops and 2 reen 

 trant angles on each side, the middle loops of the two sides not opposite ; 

 nio with anterior loop on outer side narrow and followed by a shallow 

 reentrant angle; nig shaped much like letter S : outer side with 2 prom 

 inent and strongly convex loops and 1 deep reentrant angle ; inner side 

 with a convex anterior loop, a moderately deep reentrant angle, and a 

 long flat heel which curves outward posteriorly to form posterior loop 011 

 outer side. 



The enamel pattern of the crowns of the middle upper and 1st and 2d 

 lower molars changes rapidly with wear ; that of the last lower molar 

 more slowly ; in m- the anterior reentrant angle on outer side disappears, 

 leaving a large anterior and small posterior lobe, with a small enamel 

 island in the former ; in m T and m? the anterior reentrant angle on outer 

 side and posterior reentrant angle on inner side disappear, converting 

 the crown into two large lobes not unlike a figure 8 turned sideways ; in 

 m^ the resulting shape is more like the letter S ; in m^ the reentrant 

 angle on inner side disappears with age, leaving the inner side plain. 

 The upper molars seem too large for the jaw, and the middle ones are 

 sometimes tilted out of line. 



Neotomodon alstoni sp. nov. 



Type from Nahuatzin, Michoacan, Mexico (alt. 8500 ft.). No. 50534, 

 c? old, U. S. Nat. Mus., Biological Survey Coll. Collected Oct. 12, 1892, 

 by E. W. Nelson. Orig. No. 3580. 



Characters. Size of a rather large Microtus pennsylvanicus ; ears large 

 and scantily haired ; hind feet rather long and slender; tail shorter than 

 head and body, sharply bicolor, and moderately haired; color dusky 

 grayish. N. alstoni is the largest of the three species here described. 



Color. Upper parts dusky grayish, darkened on back, and varying 

 with season to dull fulvous brown ; under parts dark plumbeous, washed 

 with white and with a rather faint huffy suffusion on pectoral region ; 

 wrists and ankles dusky, the dusky extending out a short distance on 

 metatarsus ; fore and hind feet white ; tail bicolor, dusky above, whitish 

 below. 



Cranial characters. The cranial and dental characters have been fully 

 described under the genus and need not be repeated. Compared with 

 the other known species of the genus, perotensis and orizabx, the skull is 

 larger and heavier, and when old much more angular. 



Measurements. Type specimen (^ old) : Total length 225 ; tail vertebrae 

 103 ; hind foot 28. Average of 7 specimens from type locality : Total 

 length 220 ; tail vertebrae 101 ; hind foot 26.5. 



Remarks. Specimens referred to this species were collected by Mr. 

 Nelson at Nahuatzin, Michoacan, Huitzilac, Morelos, and the following 

 places in the State of Mexico: Amecameca, Ajusco, north slope of Volcan 

 Toluca, Toluca Valley, Salazar, and Mt. Popocatapetl. 



