86 



MICROTIN^ 



process to a point just below the condyle, where their capsule 

 projects, and rises to about the level of the condyle itself. 



The cheek-teeth are rooted in adults. The enamel is rather 

 thinner than in Prometheomys and about equally thick on the 

 concave and convex sides of the salient angles. Owing partly 

 to the shallowness of the re-entrant folds, which do not contain 

 cement, and partly to the imperfect alternation of the inner and 

 outer angles, the dentinal spaces show an unusual degree of con- 

 fluence. Apart from their generic peculiarities m}, m^, and mg 

 are in pattern essentially as in normal voles, but m^ occasionally 

 shows traces of the cusp x^ and of the fold which primitively 

 separates that cusp from 6 — a very archaic character; m^ is 

 greatly reduced in much the same way as in Prometheomys, but 

 the reduction goes even further, the first outer triangle and the 



Fig. 55. — Cheek-teeth of Promelhcomys scJia poscJmikotvi Satunin. 

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



infold primitively separating it from the anterior loop being 

 reduced to microscopic vestiges, and although m^ and 7)1^ develop 

 two roots each m^ acquires but a single fang. In the lower jaw 

 Ml consists of a posterior loop, three to five alternating triangles, 

 and a short anterior loop ; its posterior loop is usually substantially 

 closed in front, but the triangles are more or less confluent with 

 each other and with the anterior loop ; the degree to which the 

 fourth and fifth triangles are difterentiated from the base of the 

 anterior loop varies with the species, the individual, and with 

 age ; ^nj is essentially as in normal voles apart from its confluence ; 

 wij is like JWg, but is a little more reduced, its antero-external angle 

 being obsolete ; m^ and ?».2 have two roots each, which pass down 

 through the jaw to the labial side of the incisor; m^ has a single 

 fang, lying on the lingual side of the incisor, but sharply curved 

 backwards to follow the dorsal curvature of the incisor, instead 

 of continuing nearly straight downwards as in all other voles 

 with the exception of Prometheomys. This abrupt backward 



I 



