98 ^Ii-. 0. S. Miller on (he 



Key to the Members of the Subgenus Chionomys. 



Pterygoids with inner faces not parallel ; width of 



mesopterj'goid fossa anteriorly much greater 



than distance from edge of fossa to alveolus of 



m\ 



Ear about 16-5 mm. ; molars with salient angles 



unusually acute, (Northern Asia Minor ; 



Caucasus ?) Microtus pontius. 



Ear about 13'6 mm. ; molars with salient angles 



not unusually acute. (Palestine.) M. hermonis. 



Pterygoids with inner faces parallel (vertical) ; 



width of mesopterygoid fossa anteriorly never 



more than equal to distance from edge of fossa 



to alveolus of m^. (Europe.) 



Hind foot iu adult less than 20 mm. ; back clear 



light grey or with very slight brownish tinge ; 



tail always white M. lebnmii. 



Audital bullse lai-ge and strongly inflated 

 (normal) ; back with slight brownish 



tinge. ( Basses- Alpes.) M. I. leucurus. 



Audital bullae small and flattened ; back clear 



light grey. (Gard.) M.J. lebrunii. 



Hind foot in adults 20 mm. or more ; back de- 

 cidedly brownish ; tail often not entirely 

 white. 

 Posterior border of palate with median ridge 

 sharply defined, its width less than that of 

 deep lateral -pit ; back strongly clouded with 



blackish ; tail usually dark above M. ulpius. 



Posterior border of palate with median ridge 

 seldom sharply defined, its width always 

 at least equal to that of shallow lateral pit ; 

 back slightly or not clouded with blackish ; 



tail usually not dark above M. fiivalis. 



Anterior loop of first lower molar with 

 postero-external salient angle usually nar- 

 row and sharply pointed, never obsolete. 



(Alps.) M. n. nivalis. 



Anterior loop of first lower molar with 

 postero-external salient angle usually 

 broad and rounded, sometimes obsolete. 

 (Pyrenees.) , M. n. aquitanius. 



Microtus nivalis (Martins). 



(Synonymy under subspecies.) 



Geographical distribution. — Pyrenees, Alps (except extreme 

 south-western portion), Apennines, and Tyrol. 



Characters. — Size large (hind foot 20 to 22 mm., condylo- 

 "basal length of skull in adults 28 to 30-4 mm.). Skull with 

 pterygoids vertical, their inner surfaces parallel ; meso- 

 pterygoid fossa narrow, its width anteriorly never more 

 than equal to distance between fossa and alveolus of third 



