392 



MURID^— LEMMUS 



anterior edge of the inter-pterygoid fossa carried forward 

 dorsally past the overhanging edge of the palate. 



In the teeth the ungrooved upper incisors are compara- 

 tively slender. The cheek-teeth (Fig. 57) are rootless; their 

 tooth-rows converge anteriorly, their crowns are very broad, 

 and the enamel-pattern is characterised by reduction of dentine- 

 spaces, sharp enamel-angles, and deeply cut infolds ; the latter 

 run almost right across m\ and m2 on the outer upper and 

 inner lower sides ; in some cases they are opposed by salient 

 angles presenting a peculiar appearance of square truncation. 



Except where modified by the features just described, 

 ni^ and n^ do not differ from those of normal Microtince. 



m^ is formed of four transverse 

 loops ; the first is isolated from 

 the second by a deep outer 

 fold ; the second from the third 

 by the approximation of an 

 inner and outer fold, the former 

 a little deeper ; the third simi- 

 larly from the fourth, but the 

 inner fold is very deep, the 

 outer a mere vestige. 



OTi has the posterior loop 

 preceded by three closed tri- 

 angles, and an anterior loop 

 formed by the blending' of 

 two or three reduced dentinal 

 spaces ; it presents three outer 

 and four inner salient angles. 

 m^ agrees in structure with that 

 of Microtus, and differs merely 

 in having the two outer closed triangles relatively small. 

 W3 is essentially similar ; its postero-internal fold is, however, 

 deepened, so that it extends practically across the crown, 

 and the antero-external triangle of m^ is not represented ; there 

 is a large foramen below and behind the alveolus. 



The above characters show extreme specialisation in skull 

 and teeth, but in external characters Lemmus is not so 

 specialised as Dicrostonyx. 



mi — 



Fig. 57. — Crown Views of Right Upper 

 (a) and Lower (b) Cheek-Teeth of 

 Lemmus limmus (7i times life size) ; from the 

 late Pleistocene of England. (Drawn by 

 M. A. C. Hinton.) 



