682 



^is/ 



Fig. 133. 

 Microtus arvalis. Xat. .size. 



exact proportion of length to breadth varying in the different 

 races ; auditory buUie frequeiitly though not always larger than in 

 31. agrestis (proportionately to size 

 of skull) ; nasals narrowing more 

 gradually backward, not abruptly 

 contracted at middle ; mandible with 

 coronoid process usually less notice- 

 ably curved backward, and articular 

 process marked on outer side by a 

 more obvious protuberance over base 

 of incisor-root. 



Teeth. — Incisors as in Microtus 

 agrestis except that the front face 

 of upper teeth is more nearly flat, 

 that is, less obliquely rounded off 

 at outer side. Molars both above 

 and below differing from those of 

 31. agrestis in a general tendency 

 toward wider re-entrant angles and 

 smaller closed triangles, which gives the pattern as a whole 

 a less compact aj^pearance. In details of enamel folding the 

 only imjiortant difference between the two 

 animals is the comj^lete absence in M. arvalis 

 of a postero-internal loop to m^. The third 

 upper molar has exactly the same elements 

 as that of 31. agrestis, and is subject to 

 similar variations in form. First lower 

 molar with re-entrant angle on inner side of 

 anterior loop usually less developed than in 

 31. agrestis, that on outer side somewhat 

 deeper, so that the two are approximately 

 equal ; this causes the loop to apj^ear to 

 project forward, or to turn outward instead 

 of inward. 



Semarl's. — Among European voles 3Iicrotus arvalis is dis- 

 tinguished by its enamel pattern combined with the perfectly 

 normal skull and small or medium size. As in the case of 

 3t. agrestis, dry sj^ecimens, particularly those that are faded and 

 distorted, cannot always be positively determined. Though 

 always conforming to the tetramerodont type common to the 

 majority of species of true Microtus occurring in both the Old 

 World and America, the enamel pattern shows a somewhat 

 unusual tendency toward individual variation in exact details of 

 form.* These variations appear to be in no way characteristic 

 of local races ; and the five subspecies here recognized are based 

 on other characters. 



* The variations in the enamel pattern have been studied and figured 

 in great detail by Rorig and Boruer, Arbeiten aus der Kaiserlichen Biolog- 

 ischen Anstalt fiir Land- und Forstwirtschaft, v, Heft ii, i^p. 37-89, 

 pis. iv-vi, 1905. 



FIG. 134. 



ifierottis arvalis. 



Enamel pattern, x S. 



