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 m, 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 w* and mz. 
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 mm, 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 
