682 



RODENTIA 



Fig. 133. 

 Microtia arvcUia. Nat. size. 



exact proportion of length to breadth varying in the different 

 races ; auditory bullae frequently though not always larger than in 



M. 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 

 M. agrestis in a general tendency 

 toward wider re-entrant angles and 

 smaller closed triangles, which gives the pattern as a whole 

 a less compact appearance. In details of enamel folding the 

 only important difference between the two 

 animals is the complete absence in M. arvalis 

 of a postero-internal loop to m 2 . The third 

 upper molar has exactly the same elements 

 as that of M. 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 

 M. agrestis, that on outer side somewhat 

 deeper, so that the two are approximately 

 equal ; this causes the loop to appear to 

 project forward, or to turn outward instead 

 of inward. 



Bemarhs. — Among European voles Microtus arvalis is dis- 

 tinguished by its enamel pattern combined with the perfectly 

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

 M. agrestis, dry specimens, 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 Eorig and Borner, Arbeiten aus der Kaiserliohen Biolog- 

 iaohen Anstalt fur Land- und Forstwirtschaft, v, Heft n, pp. 37-89, 

 pis. iv-vi, 1905. 



FIG. 134. 



Mierotus arvalis. 

 Enamel pattern. X 5. 



