OS Miller Description of a New Vole from Oregon. 



Skull. Fully grown skulls vary from 22.6 to 24.6 mm. in basilar length 

 and from 14.6 to 15.6 in zygomatic breadth ; brain case broader and 

 deeper than in M. nanus ; andital bullae flatter and rounder; bony palate 

 with excessively shallow lateral pits. 



Teeth. The enamel pattern, like that of the other members of the nanus 

 group, is that of the typical or tetramerodont species of the subgenus 

 Microtus. 



Measurements. Type specimen: total length, 135; tail vertebrae, 33; 

 hind foot, 20. Average of eight adult topotypes: total length, 141 ; tail 

 vertebrae, 35.7; hind foot, 20. (All measurements from fresh specimens 

 by collector. ) 



General remarks. Microtus nanus, M. canicaudus, and M. mogoUon<i<xix 

 form a group of closely related species which in size, general appearance, 

 and cranial and dental characters differ noticeably from other American 

 members of the genus, but strikingly resemble the European Microtus 

 arvalis. So close is this resemblance that it is possible to select skulls of 

 M. mogollonensis (the smallest member of the group) from San Francisco 

 Mountain, Arizona, that are practically indistinguishable from skulls of 

 M. arvalis taken in Slavonia. The only constant cranial characters to 

 distinguish the skulls of these two species appear to be the slightly broader 

 rostrum, more flaring zygomata, and wider, less squarely truncate in- 

 terpterygoid fossa of mogollonensis. Externally the resemblance is no less 

 close, for M. arvalis has the same short tail and small hind foot as the 

 American species, while in color it differs only in a somewhat yellower 

 cast. 



The three American species are distinguished from each other by the 

 following characters : 



M. mogollonensis (Mearns). Total length, 132.2; tail vertebrae, 28.3; 

 hind foot, 18 (average of eight adults) ; general color grizzled yellowish 

 brown; tail indistinctly bicolor; basilar length of skull about 21 milli 

 meters ; nasal branches of premaxillaries considerably extended back of 

 nasals ; audital bullaa roundish ; bony palate with lateral pits very deep. 



M. nanus (Merriam). Total length, 140 ; tail vertebrae, 37.7 ; hind foot, 

 18.9 (average of nine adults) ; general color grizzled grayish brown; tail 

 distinctly bicolor; basilar length of skull about 23 millimeters; nasal 

 branches of premaxillaries slightly extended back of nasals; audital 

 bullae subfusiform ; palate with lateral pits moderate. 



M. canicaudus Miller. Total length, 141 ; tail vertebrae, 35.7 ; hind foot, 

 20 (average of eight adults) ; general color uniform umber-brown ; tail 

 indistinctly bicolor; basilar length of skull about 23 millimeters; nasal 

 branches of premaxillaries scarcely extended back of nasals ; audital bullse 

 roundish ; palate with lateral pits very shallow. 



