﻿64 



NORTH AMERICAN FAUNA. 



[Xo. 12. 



imens of Microtus agrestis, M. xjennsylv aniens^ also iu tlie type of M. 

 {Arvicola) arvicoloides (tig. 35), aud probably in any other species with 

 the tootli formed after tlie pattern of Microtus arvalis. The other varia- 

 tions in the form of the front lower molar are the resnlt of the greater 

 or less development of the reentrant angles normally present at the 

 anterior end. Sometimes the fonrth reentrant angle (connting from 

 behind) on the liiignal side of the tooth fails to meet the third on the 

 ojiposite side. Very rarely the anterior onter triangle opens in a like 

 manner into the anterior inner triangle, and the latter at the same time 

 commnnicates with the anterior loop, thus producing a tooth like that 

 normally present in Pedonii/s and Fitymys. Eather frequently a sixth 

 closed triangle is cut off from the outer basal corner of the anterior loop, 

 and occasionally a seventh triangle is isolated at the inner side of the 

 greatly reduced loop. 



The variations just described are purely individual and occur in the 

 species having the tooth of the typical form. Two notable variations 

 from this form are normally found in Microhis ratticeps and 21. nivalis. 

 In the former (tig. 33r?) the fifth triangle opens into the short, unindented 

 anterior loop. There is here an actual reduction in the elements of the 

 ooth, which has only eight salient angles, thus resembling the corre- 

 sponding tooth in Fedomys. In 21. nivalis (fig. 33h), while there are five 

 closed triangles and nine salient angles, the anterior loop is small and 

 crescentic, much resembling the posterior loop in the maxillary teeth 

 of Eotlienomys, 



Bad' npper molar. — The last upper molar is normally made up as 

 follows: An anterior transverse loop, succeeded hj three closed trian- 

 gles, two smaller ones on the outer side and a larger one on the inner 

 side, these in turn by a i^osterior loop of variable shape. The tooth 

 usually contains seven salient angles, two to each of the transverse 

 loops and one to each of the three closed triangles. 



Variations in the form of this tooth are numerous. Beginning at the 

 anterior end where the structure is most definite, it is found that the 

 first outer triangle very frequentl}^ opens into the large inner trian- 

 gle, less often into the anterior loop. The second outer triangle very 

 rarely opens into the inner triangle, but is rather frequentl}^ in commu- 

 nication with the posterior loop. The posterior loop varies in form and 

 size, the variations being i^artl}^ individual and partly characteristic of 

 species. For the present it is unnecessary to discriminate in all cases 

 between the two categories. The most usual form and that found in 

 the type species, 2Iicrotus arvalis (fig. o3a)is an irregular crescent with 

 the concavity directed inward aud backward and the posterior tip thick- 

 ened, the whole joined to the rest of the tooth at a point on the con- 

 vexity midway between the middle and the anterior extremity. This 

 nearly crescentic form is usually distorted by the elongation and straight- 

 ening of the anterior limb, so that the resulting shape is more like that 

 of the letter J. The thickened i^osterior extremity of the loop is often 



