MICROTUS 427 



sloping dorsally between two lateral pits ; this ridge becomes 

 narrower and more abrupt with age ; it may sometimes 

 encroach on the anterior edge of the inter-pterygoid fossa, or 

 it may be slightly cleft centrally. 



In the teeth the root of each lower incisor crosses the tooth- 

 row to the labial side between nti and m^, displacing the latter 

 tooth and forming a more or less distinct protuberance on the 

 outer surface of the condylar process of the mandible. 



The cheek-teeth are permanently rootless ; growing con- 

 tinuously from a persistent pulp/ they do not wear away with 

 age. Their enamel-pattern is characterised by the substantial 

 closure of all triangles in adult stages of wear, by acute salient 

 angles, and by the large number of salient angles in m^ and m■^. 

 m^ usually consists of an anterior transverse loop succeeded by 

 three closed triangles, two smaller on the outer and a larger on 

 the inner side, followed by a posterior loop of variable shape. 

 There are at least seven salient angles (four on the inner 

 side), two formed by the anterior transverse loop, one by 

 each of the three closed triangular spaces, and two by the 

 posterior loop. 



In Ml there are usually a posterior transverse loop ; five 

 closed triangles — two on the outer and three on the inner side ; 

 and an anterior loop, usually more or less deeply cut by an 

 inner and an outer fold, the latter always posterior to the former. 

 There are in normal species seven infolds and at least nine 

 salient angles ; two of the latter are formed by the posterior 

 transverse loop, one by each of the five closed triangles, and one 

 by each side of the base of the anterior loop. 



Variations from the normal enamel-pattern of the genus 

 occur constantly, and characterise the dentition of several 

 species ; in addition there may exist in any species variations 

 of a purely individual nature, often due to varying stages of 

 tooth- wear. In m^ the first outer triangle may open into the 

 large inner, or less frequently into the anterior loop. The 

 second outer triangle may rarely open into the inner, but 

 rather frequently communicates with the posterior loop. The 

 posterior loop is of quite variable shape. In m-^ there may be 

 a sixth, occasionally a seventh, closed triangle, in each case cut 

 off from a much reduced anterior loop. Sometimes the fourth 



