106 M&rriam — Neiv Microtus from Mexico and Guatemala. 



from Todos Santos, Guatemala. Both localities are considerably 

 farther south than the southernmost published record of any 

 member of the genus. 



Subgenus MICROTUS Schrank. 



Type, Microtus ur calls (Pailas) from Europe. 



Microtus fulviventer sp. now 



Type from Cerro San Felipe, Oaxaea, Mexico (alt. 10200 ft,). No. 68250, 

 c? ad., TJ. S. Nat. Mus., Biological Survey Coll. Collected Aug. 22, 1894, 

 by E. W. Nelson and E. A. Goldman. Orig. No. 6601. 



Characters.— Similar to M. mexicanus in size and general characters, but 

 upper parts very much 'redder' and under parts fulvous instead of 

 whitish ; tail short ; ears conspicuous. 



Color. — Upper parts uniform dark umber brown mixed with black 

 hairs; under parts dark fulvous or chestnut-fulvous ; tail indistinctly bi- 

 color, blackish above, pale fulvous below, darkening toward tip. 



( 'ranial and dental characters. — Skull and teeth similar to those of M. mex- 

 icanus, hut slightly larger; interorbital region broader; incisive foramina 

 longer ; molars heavier. 



Measurements. — Type specimen: Total length 154; tail vertebrae 38; 

 hind foot 20. 



Subgenus ORTHRIOMYS* nobis. 



Type, Microtus umbrosus sp. nov., from Mt. Zempoaltepec, Oaxaea, 

 Mexico. 



( 'haracters. — Palate normal ; interorbital constriction rather broad ; m T 

 with 3 closed triangles (2 on inner and 1 on outer side) and 2 open trian- 

 gles, the latter forming the wings of the anterior trefoil ; m 3 with 2 com- 

 pletely closed triangles (1 on each side) and 2 obliquely transverse loops 

 (anterior and posterior) both on inner side; the outer triangle immedi- 

 ately followed by a deep reentrant angle which completely cuts it off from 

 inner triangle ; m^ with 1 closed triangle on each side. Mammae 4: pec- 

 toral | = 4. Fur long and soft, only extreme tip colored.. 



Remarks. — In the type and only known species, M. umbrosus, m 1 has 

 only 2 closed triangles and a posterior open trefoil ; the molars are very 

 broad and heavy, the breadth relatively greatest posteriorly ; the enamel 

 loops and triangles are very large, full, and broadly rounded, enclosing 

 large islands of dark osteodentine, and so crowded as to exceed the inter- 

 spaces. The crowns of the molars resemble those of Phenacomys in the 

 polish of the enamel, darkness of the osteodentine and of the crowns as 



* opOpios, early; [w<s, mouse. In main' respects Orthriomys suggests an 

 ancient type intermediate between Phenacomys and the microtine sub- 

 genera Pedomys and Arvicola; in the character of its molar crowns it is 

 nearest Phenacomys. 



