New Microtus from Mexico and Guatemala. 107 



a whole (contrasted with the whitish molars of Microtus) ; large size and 

 fullness of the loops and triangles (in this respect exceeding Plienacomys) ; 

 pyriform shape of anterior loop in m^ and m^ ; enamel pattern of upper 

 molars, including m^ ; only slightly modified enamel pattern of lower 

 molars, and great breadth of molar series (above and below) posteriorly. 

 They differ from those of Plienacomys in having the outer triangle of m j 

 completely isolated ; two less triangles on m 1? less disparity in depth be 

 tween the reentrant angles of the two sides in the lower molars, and in 

 the total absence of roots. 



Microtus umbrosus sp. nov. 



Type from Mt. Zempoaltepec, Oaxaca (alt. 8200 ft.). No. 68480, 9 ad., 

 U. S. Nat. Mus., Biological Survey Coll. Collected July 10, 1894, by 

 E. W. Nelson and E. A. Goldman. Orig. No. 6412. 



Characters. Size medium or rather large ; tail long and scantily haired ; 

 ears rather short and nearly concealed by fur ; color very dark ; fur long, 

 soft, and full ; dentition peculiar. 



Color. Upper parts uniform dusky with tips of hairs brown; under 

 parts dark slate, washed with pale dull fulvous. 



Cranial and dental characters. Skull rather long ; braincase long ; ante 

 rior roots of zygomata not notched in front ; zygomata not widely spread 

 ing, sides parallel ; jugal not expanded ; audital bullse small : palatine 

 pits deep ; postpalatal notch square ; interorbital constriction broad ; 

 premaxillse reaching posteriorly considerably beyond nasals ; nasals nar 

 rowing posteriorly; incisive foramina rather short. Molars large and 

 broad ; m^ with 1 closed triangle on each side, and a short posterior tre 

 foil or v presenting one open salient angle on each side [remaining teeth 

 described under subgenus]. 



Jlfeasurements. Type specimen: Total length 184; tail vertebrae 65; 

 hind foot 23. Average of 7 specimens from type locality : Total length 

 177 ; tail vertebne 61 ; hind foot 23.5. 



Subgenus HERPETOMYS * nobis. 

 Type, Microtus gaatemaiensis sp. nov., from Todos Santos, Guatemala. 



Characters. Palate normal ; m T with 3 closed triangles (2 on inner and 

 1 on outer side) and 2 open triangles, the latter forming the wings of the 

 anterior trefoil ; mj with 2 completely closed triangles (Ion each side) 

 and 2 obliquely transverse loops (anterior and posterior) both on inner 

 side ; m- with 3 closed triangles (2 on outer and 1 on inner side) and a 

 long posterior crescentic loop with both horns projecting on inner side. 

 Mammse6: pectoral f, inguinal { (the latter not functional). Plantar 

 tubercles 5. Fur long and soft with only extreme tip colored. 



Itemarks. This animal, while agreeing with Orthriomys in the number 

 of enamel loops and triangles of the lower molars, differs strikingly in 



j creeper ; fifi^ mouse, 



