246 



MICROTIN^ 



side (Fig. 8\d) ; in young stages of wear the dentinal sjoaces are 

 more confluent, the posterior loop longer, and there are more or 

 less clear vestigial traces of one or two extra pairs of salient angles 

 behind (Figs. 81a ; 78 iia). In ?», the five triangles following the 

 posterior transverse loop are usually substantially closed in adults, 

 but the anterior or fifth triangle is sometimes narrowly confluent 

 with the small anterior loop ; in youth ephemeral complications of 

 the anterior loop of this tooth are sometimes visible (Fig. 78, i, 2), 

 and the dentinal spaces are commonly more confluent with each 



Fig. 81. — Evotomys rufocanu6 Sund. Upper Molars. 

 Explanation of Figs. 81 and 82 : a-d. E. r. smithii (Japan) ; a, a'. B.M. 

 No. G.1.4.365, young ; 6, h'. 5.3.3.49, young {type) ; c, c'. 6.1.4.307, subadult 



other. In m^ there is also a tendency to close two or more of the 

 four triangles in front of the posterior loop (Fig. 82, c', d'). Lastly, 

 in extreme old age, when the basal parts of the crowns are exposed 

 by wear, the pattern degenerates, the teeth acquiring again both 

 confluent dentinal spaces and rounded salient angles similar to 

 those of adult teeth in normal species of Evotomys. 



Geographical differentiation. — A careful study of the material 

 in the British Museum and of the literature leads me to believe 

 that E. rufocanus as generally understood and all the many 

 nominal species of " Craseornys " and " Phaulomys " that have 

 been described in recent years from the Far East and from Central 

 Asia must be referred to a single sisecies. Ou the one hand, like 



