388 



MURIDiE— MICROTIN^ 



three infolds, and three inner salient angles with two infolds. 

 In some species of Dicrostonyx {D. torquatus and the extinct 

 British D. gulielmi) vi- and m^ have, in addition to the parts 

 enumerated above, a minute vestigial postero-internal salient 

 angle (Fig. 55, «^) — the last trace of what may once have been 

 an important element in each tooth, ni is essentially like m^ in 

 form, differing merely in the degree of reduction of its posterior 

 portion ; the first five dentinal spaces are like those of v^ ; the 

 fourth outer angle— the vestigial one in the anterior teeth— is 

 here largely developed, as is also, on the inner side, the 



homologue of the minute postero- 

 internal angles of w}- and n^ of D. 

 torquatus asidgulielmi ; these two 

 triangles are incompletely sepa- 

 rated from each other by the third 

 inner fold, and together they form 

 a crescentic posterior loop; briefly, 

 this tooth has six or seven dentinal 

 spaces, with four salient angles 

 and three infolds on each side. 



The lower cheek-teeth of 

 Dicrostonyx (as also of other 

 Murotincs) are in general like 

 those of the upper jaw, but the 

 transverse loop is at the posterior 

 instead of at the anterior end of 

 each tooth (Fig. 55, b). Wj con- 

 sists of a posterior transverse loop, 

 preceded by seven substantially closed triangles, of which four, 

 including the anterior, are internal and three external ; anterior 

 of all is a loop of complex structure, into the composition of 

 which some four or more originally distinct cusps enter ; 

 occasionally one or two of the posterior of these may be com- 

 pletely separated off from the more forward part of the loop 

 by infolds in the usual manner, thus increasing the number 

 of closed triangles to eight or nine ; there are never fewer 

 than nine dentinal spaces, five outer and six inner salient 

 angles, and four outer and five inner infolds. The two 



— mi — 



Fig. 55. — Crown View of Right 

 Upper (a) and Lower (b) Cheek- 

 teeth OF Dicrostonyx torquatus. Drawn 

 by M. A. C. Hinton (7i times life size). 



posterior teeth, essentially like each other in form, are 



