716 RODENTIA 



SO that the second loop is often nearly or completely divided into 

 two closed triangles. Third upper molar with anterior loop and 

 three closed triangles of the usual form, but posterior loop short, 

 simple and without trace of re-entrant angle on inner side 

 (though in some instances a slight widening of the extreme tip 

 causes a certain degree of concaveness), its form much like that 

 assumed by extreme terminal portion of loop in 31. agrestis when 

 second inner triangle has been cut off ; the tooth never has more 

 than three well developed salient and two re-entrant angles on 

 each side. First lower molar essentially as in Microtus ratticeps, 

 the oiiter side with three, the inner side with four re-entrant 

 angles, the anterior loop short, simple, without trace of re-entrant 

 angles, its general outline crescentic (or when more elongate 

 almost arrow-head shaped) when outer basal portion is well 

 developed, or unsymmetrical when this is short or obsolete ; first 

 inner triangle rarely communicating with anterior loop ; outer 

 side normally with four salient and three re-entrant angles, inner 

 side with five salient and four re-entrant angles. 



Remarks. — Microtus nivalis is a strictly alpine animal, never 

 inhabiting plains or true lowlands and only descending into 

 valleys when special local conditions, such as damp, shaded clilfs 

 or cool talus-slopes,* furnish it with the low summer temperature 

 which it requires. Two local forms are distinguishable, one 

 confined to the Pyrenees, the other to the region of the Alps. 



Microtus nivalis nivalis Martins. 



1842. Arvicola nivalis ilartins, Ecvue Zoologique, p. 331 (Faulhoni, Bern, 



Switzerland). 



1843. Hyimdseus aljyinus Wagner, Schreber's Siiugthiere, Supp]., iii, p. 57G 



(Andermatt, Uri, Switzerland). 

 1845. Arvicola nivicola Schinz, Syuops. Mamm., ii, p. 236 (Highest Swiss 



Alps; probably near Andermatt). 

 1853. Hypucheus pctrophilus Wagner, Miinch. Gelehrt. Anzeigen, No. 38, 



p. 307, March 28, 1853 (Oberstdorf, near Sonthofen, Allgau, 



Bavaria, Germany). 

 1857. Arvicola nivalis Blasius, Saugethiere Deutschlands, p. 359. 

 1884. \_Microtus] nivalis Lataste, Actes Soc. Linn., Bordeaux, xxxviii, 



p. 23 (p. 15 of reprint). 

 1908. Microtus nivalis nivalis Miller, Ann. and l\Iag. Nat. Hist., Sth ser., 



I, p. 99, January, 1908. 

 1910. Microtus (Chionomys) nivalis Troucssart, Fanne ]\Iamm. d'Europe, 



p. 183. 



Type localitjj. — Faulhorn, Bern, Switzerland. 



Geofjraplilral distribution. — Alps (except south-eastern portion), 

 Tirol and northern Apennines. 



Diagnosis. — Anterior loop of first lower molar tending to 

 assume an arrow-head-like outline, owing partly to the general 



* For a study of the local conditions in a typical talus-slope see Miller, 

 Science, N.S., viii, pp. G15-618, November 4, 1898. 



