112 



MICROTIN-ffi 



complicating the anterior border of the prism. It is interesting 

 to note that this particular median tubercle (always, of course, 

 associated with the third outer angle) crops up again and again 

 as an atavism in many widely separated Microtine genera (cf. 

 Fig. 67, 3). The anterior loop proper is too far worn in the 



Fig. 60. — Mimomys plioccenicus Forsyth Major. 



Crown and external views of the nii ia different stages of wear 

 (Norwich Crag). 



a, a'. Right Wj, cement spaces closing. 



b, h'. Right »i,, cement spaces closed. 



c, c'. Left TBj, with short roots. 



d. Left TO], with moderately long roots (for outer and inner views see 



PI. XIII, figs. 1, la). 



e. Right »ij, old, with very long roots. 



subject of Fig. 60a to show any clear trace of complications, 

 although a certain waviness of its antero-external border suggests 

 that these were present in a slightly earlier stage of wear. 

 One other feature deserves notice, namely, the peculiar outward 

 convexity of the isthmus connecting the posterior loop with the 



