70 



MICROTIN^ 



in m^ and m- of distinct vestiges of one or more of the " inter- 

 mediate " tubercles ; m^ and m^ apart from the generic peculiarities 

 are normal, but the second inner fold of m^ and the inner fold 

 of m- are usually complicated by a vestige of the " protoconule " 

 (cusp y) ; m^ is more or less simplified, with three or four outer 

 and two or three inner salient angles. In general appearance 

 the upper molars, particularly m^, show a strong likeness to those 

 of Alticola. In the mandible w^ has five closed triangles between 

 the posterior and anterior loops as in Microtus ; m^ and m^ are 

 normal, but the last-named tooth has its outer angles and infolds 

 less reduced than usual. 



Notwithstanding its high specialization for aquatic life 

 Ondatra, peculiar to North America, is apparently more closely 

 related to Phenacomys than to any other Microtine genus, and 

 it has descended from some primitive Phenacomys-like stock. 

 In one respect, the palate, it is even a little more primitive than 



Fig. 43. — Cheek-teeth of Lagurus luteus Eversmann. 

 Crown views : a. left upper, 6. right lower molars. 



is any known Phenacomys. The cheek-teeth are distinctly 

 primitive and Phenacomys-like ; they develop roots when adult 

 and have enamel of about equal thickness on the concave and 

 convex sides of the salient angles ; the re-entrant folds are partly 

 filled with cement and close the dentinal spaces rather tightly ; 

 the depth and narrowness of these folds, coupled with the great 

 breadth of the crowns, impart to the teeth an appearance of 

 longitudinal crowding. The enamel pattern of m^, m^, and m^, is 

 normal ; m^ has three or four salient angles on each side ; m^ is a 

 complex tooth with a posterior loop, five to seven closed triangles, 

 and a short anterior loop which shows remarkable ephemeral 

 complications when unworn (see p. 115). In m^ the outer salient 

 angles are somewhat reduced in size, but the outer infolds are 

 deep, so that the first pair of triangles following the posterior 

 loop are substantially closed, and the second pair are not more 

 widely confluent than are the corresponding triangles of W2- 

 Obscure traces of " intermediate " tubercles are frequently to be 

 seen in adult or well-worn teeth. 



