AEVICOLA 



729 



goids noticeably diverging ; ectopterygoid pit large and deep, not 

 encroached on by bulla. The mandible does not differ materially 

 from that of Microtus agrestis except in its conspicuously greater 

 size, but the articular process is more abruptly bent inward at 

 level of incisor root, and angular process is more widened 

 posteriorly. 



Teeth. — Upper incisor with shaft less curved than that of 

 Microtus agrestis, its outline from tip to base less nearly a semi- 

 circle ; root at anterior margin of anteorbital foramen, where it 

 forms a slight though evident swelling. Cross-section of shaft 

 essentially as in If. agrestis, but antero-posterior diameter 

 relatively greater, so that inner border is slightly longer than 

 anterior border ; distribution of enamel as in the smaller animal. 

 Lower incisor with root extending well into base of articular 

 process, on the outer side, of which it 

 usually forms a slight protuberance ; cross 

 section, like that of upper tooth, with 

 antero-posterior diameter relatively greater 

 than in Jlf. agrestis, the enamel-covered 

 anterior surface thus relatively narrower. 

 Molars large and heavily built ; m 3 form- 

 ing a conspicuous capsule on inner side 

 of incisor, the roots of m 1 and m 2 appear- 

 ing as distinct protuberances on lower 

 surface of mandible. Enamel pattern 

 well denned, all the elements distinct, the 

 angles sharp and definite. Details of 

 pattern much as in Microtus nivalis ; m 1 

 with anterior transverse loop and four 

 alternating, essentially equal closed tri- 

 angles, each side with three salient and 

 two re-entrant angles ; m 2 with anterior 

 transverse loop and three alternating closed 



triangles, the outer side with three salient and two re-entrant 

 angles, the inner side with two -salient and two re-entrant 

 angles ; rn 3 with anterior transverse loop, a larger inner and 

 smaller outer closed triangle, and a short longitudinal loop, 

 from the antero-external base of which a small third closed 

 triangle is sometimes cut off; each side of tooth with three salient 

 and three re-entrant angles, the last two salients occasionally ill- 

 developed in individuals with reduced terminal loop ; m 1 with 

 same elements as in Microtus nivalis, bmt first and second 

 triangles normally opening into each other and into short, sub- 

 terete anterior loop, of which in some specimens they appear to 

 be mere angular basal appendages ; true closed triangles only 

 three, one outer and two inner ; outer side of tooth with four 

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

 rudimentary fifth (anterior) salient and four re-entrant angles ; 

 m 2 with four closed, sub-equal triangles (the antero-internal and 



Fig. 161. 



Arvicola amphibius. 

 Enamel pattern. X 5, 



