MUBIDJ3— AUVICOLIN.E— ARVICOLA RIPARIUS. 159 



enamel, the lobe is cut off from the rest of the trefoil, and transformed into 

 an actual closed external triangle, of which, therefore, the tooth may show 

 indifferently two or three. A step further in modification is this: — the trefoil, 

 after being partitioned off as just explained, may develop an external lobe on 

 its midleaf, and so remain in effect a trefoil, giving in all five external salien- 

 cies, viz: one from the posterior loop of tbe tooth; two from the two closed 

 triangles proper; one from the additional closed triangle; and one from the 

 supplementary lobe of the trefoil.* We have not noticed the same thing 

 with the inner leaflet of the trefoil, but presume it may occur. 



The upper molars reverse the pattern of the lower; their transverse loop 

 is in front instead of behind, and the lateral triangles alternate from before 

 backward instead of from behind forward; and the last upper molar ends 

 behind, just as the first lower molar begins in front, with a formation not 

 seen on any other teeth. The first upper molar, like the last under molar, 

 is very constant throughout the whole genus; the last upper and first under 

 are our diagnostic teeth for the several subgenera, though the middle ones, 

 both upper and under, furnish collateral characters. 



The front upper molar has in front a transverse loop, succeeded by two 

 internal and two external alternating lateral closed triangles; the first of these 

 being internal, the last postero-exrernal. The saliencies and reentrances on 

 both sides are sharp; the saliencies, both external and internal, are three in 

 number, counting a corner of the anterior loop in each enumeration. 



The middle upper molar likewise consists of an anterior loop succeeded 

 by alternating lateral closed triangles, but here the first is an exterior instead 

 of an interior one. The anterior ellipse usually sits a little oblique, its con- 

 vexity looking inwai-d as well as forward. Regularly there are only two 

 perfectly-closed lateral triangles on this tooth ; first an exterior, then an inte- 

 rior, for the last one is as much posterior as interior, and not often regularly 

 triangular. It may be called an exterior triangle, appendaged with a 

 posterior lobe or spur representing an imperfect additional internal triangle. 



* This is our view of tbe formation of a variable number of lateral closed triangles iu tbis tootb ; 

 it is simply a higher degree of complexity iu the folds of the anterior trefoil. Iu Pedomys and Pilymys, 

 the reverse, of less complexity, is seen. Here the median zigzag of enamel does not run so far forward ; 

 tbe anterior triangles are not fairly closed up, and their openness throws their dentiue islets into one 

 diamond-shaped area that is usually also continuous with the interior of the trefoil itself. This is 

 nearly as good a character for distinguishing Pedomys or Pitymys from any style of A. ripariua as that 

 afforded by tbe back upper molar itself; though it is sometimes obscure, we have never seen the anterior 

 lateral triangles fairly open in riparius, nor completely closed in the other two subgenera. Chilotus, 

 however, is like riparius in this respect. 



