MICEOTUS 



713 



KEY TO THE EUROPEAN MEMBERS OP THE SUB-GENUS 

 CHIONOMYS. 



Hind foot in adults less than 20 mm. ; back clear 

 light grey or with very slight brownish tinge ; 



tail always white throughout M. lebrunii, p. 718. 



Back clear light grey ; auditory bullae small and 

 flattened (Neighbourhood of Nimes, Gard, 



Prance) M. I. lebrunii, p. 719. 



Back with slight brownish tinge ; auditory bullae 

 large and strongly inflated (normal) (Basses- 



Alpes) M. I. leucurm, p. 722. 



Hind foot in adults 20 mm. or more ; back decidedly 



brownish ; tail often not entirely white. 



Posterior termination of palate with median ridge 



sharply defined, its width less than that of 



deep lateral pit ; back rather heavily clouded 



with blackish ; tail usually dark above (Tran- 



sylvanian Alps) M. ulpius, p. 723. 



Posterior termination of palate with median ridge 

 usually not sharply defined, its width always 

 at least equal to that of shallow lateral pit ; 

 back slightly or not clouded with blackish ; 



tail not usually dark above M. nivalis, p. 713. 



Anterior loop of first lower molar with postero- 

 external salient angle usually narrow and 

 sharply pointed, never obsolete (Alps and 



Apennines) M. n. nivalis, p. 716. 



Anterior loop of first lower molar with postero- 

 external salient angle usually broad and 

 rounded, sometimes obsolete (Pyrenees) M. n. aquitanius, p. 717. 



MICROTUS NIVALIS Martins. 

 (Synonymy under subspecies.) 



Geographical distribution. — Pyrenees, Alps (except extreme 

 south-westei'n portion : Basses-Alpes and Yar), Apennines and 

 Tirol. 



Diagnosis. — Size rather large (head and body about 130 mm. 

 in adults, hind foot, 20-22 mm., condylobasal length of skull, 

 28-30 • 4 mm.); tail about half as long as head and body ; colour 

 of upper parts in adults grey, strongly suffused with bister, but 

 never much clouded with black ; tail whitish, usually tinged with 

 brown above, though never sharply bicolor throughout ; general 

 characters of skull as in the sub-genus ; mesopterygoid fossa 

 narrow, its width anteriorly never more than equal to distance 

 between edge of fossa and alveolus of m 3 , its outer borders 

 vertical ; posterior border of palate with median ridge usually 

 flattened and ill-defined, its width at least equal to that of 

 lateral pit. 



External characters. — General appearance more murine than 

 in the other European species of Microtus, owing to the long 

 tail and large ears rather than to any special modification in 

 form of body. Head relatively longer and more pointed than 



