INTERRELATIONSHIPS OF GENERA 39 



incisors are very broad and have the enamel brightly stained 

 throughout in Synaftoimjs ; but in Mictomys they are more 

 slender and the staining is weaker, disappearing towards the 

 outer edge of the tooth ; in both subgenera each incisor bears a 

 groove near its outer border in front, a memorial of the former 

 cuspidate condition of these teeth. In correlation with the differ- 

 ence in the upper incisors, the rostral part of the skull is stouter 

 in Synaptomys than in Mictomys. The cheek-teeth have essen- 

 tially the same patterns as those of Lemmus, but they present, 

 frequently and persistently, traces of the median row of tubercles 

 (y, z, etc.), and these traces indicate that although the tubercles 

 in question have blended with the so-called principal cusps they 

 are of large size and therefore of much importance. In m^ and 

 m^ the enamel folds are a little more transverse in direction than 

 in Lemmus and consequently they appear to be a little deeper ; 

 in Mictomys these teeth show a vestige of cusp n, but in Synap- 

 tomys the postero-internal corner of each tooth is rather more 

 reduced than in Lemmus. In both subgenera m^ is peculiar, 

 having the second transverse loop separated from the third 

 principally by the very deep second outer fold, the first inner 

 fold being very slightly developed. In the lower molars of 

 Mictomys the outer folds are so slightly developed that the teeth 

 have crenulate outer margins and no closed triangles; but in 

 the lower molars of Synaptomys the outer folds are deep, closing 

 off external triangles as in Lemmus. In general the molars of 

 Mictomys are more primitive than those of Synaptomys, since 

 they retain more of the primitive transverse arrangement, whereas 

 those of Synaptomys have acquired more of that alternation of 

 inner and outer elements which is perfected in Lemmus. 



In the genus Synaptomys the skull is less specialized than 

 in Lemmus. The tips of the nasals project in front of the 

 incisors. As in Lemmus the zygomatic arches are widest an- 

 teriorly, but their expansion is not so great; the planes of 

 their outer surfaces are nearly vertical instead of being con- 

 vergent dorsally, and the upper borders of the jugals are much 

 less boldly convex. The post-glenoid part of the braincase is 

 not shortened ; the braincase is therefore longer and narrower, 

 as well as less massive. Most of these characters are due to 

 weaker temporal muscles and the absence of extreme fossorial 

 specialization. The palate posteriorly is as in Microtus; in the 

 subgenus Synaptomys it is much as in Microtus arvalis, but in 

 Mictomys the posterior pits are extended at the expense of the 

 median septum, which has become long and thin as in " Steno- 

 cranius," and of the postero-lateral bridges, which are slender 

 and incomplete. The molars are considerably narrower in 

 proportion to their length than in Lemmus, but they diverge 

 posteriorly as in that genus. The pterygoid fossae are a little 

 longer than in Lemmus, but rather shallow, their floors being 

 scarcely dorsal to the ventral surface of the basisphenoid. 



