38 MICROTIN^ 



The true lemmings, Synaptomys, Myopus and Lemmus, form 

 an interesting group which, stands on a higher plane than Dicro- 

 stonyx. In the shortness and wholly lingual course of the lower 

 incisors the members of this group agree with Dicrostonyx and 

 differ from the rest of the Microtinae, and when most modified 

 the external form shows a specialization which in some respects 

 is analogous to that seen in Dicrostonyx. But in other essential 

 respects the group differs as widely from Dicrostonyx as it does 

 from the voles. The persistently growing cheek-teeth are char- 

 acterized by the reduction and loss of many of their primitive 

 terminal elements, so that their patterns are as simple longitudi- 

 nally as in the highest voles ; they are further characterized by the 

 reduction of the outer salient angles in lower and of the inner salient 

 angles in upper teeth ; by the tendency of the outer folds above, and 

 the inner folds below to retain their original character of transverse 

 valleys ; by their great breadth, by well-difierentiated enamel, and 

 by the presence of cement in the re-entrant folds. In correlation 

 with the more highly specialized molars, the anterior portions of 

 the temporal muscles have apparently gained in strength and they 

 produce more marked effects upon the portions of the skull 

 directly under their influence ; thus the post-orbital tendon stimu- 

 lates the squamosal on each side to develop a long, shelf -like, post- 

 orbital crest ; the superciliary ridges fuse sooner or later to form 

 an interorbital crest; the interorbital region itself is much 

 constricted and the squamosals, in the adults of the highest forms, 

 tend to meet anteriorly. 



Of the three genera of this group Synaptomys is the most 

 primitive. Externally it differs little from such voles as Evotomys 

 and wholly lacks the outward specialization for fossorial habits 

 or a boreal habitat which characterizes the recent species of 

 Lemmus ; the large ears and the, for a lemming, long and thinly 

 clad tail are particularly noticeable. Other primitive character- 

 istics are shown in the general lightness of the skull and but slight 

 modification of the zygomatic arches, in the large anterior 

 palatal foramina, in the opisthodont and grooved upper incisors, 

 and in the persistence of vestiges of the middle row of tubercles 

 in the molars of some species. On the other hand the posterior 

 border of the palate, the floor of the braincase, and the auditory 

 bullae show specializations which lead away from Myopus and 

 Lemmus and afford parallels with those seen in some of the higher 

 voles. 



Of the two subgenera, Mictomys and Synaptomys, into which 

 the genus is divided, Mictomys is the more primitive in some 

 respects. In it the lower incisor ends on the lingual side of the 

 molars opposite the posterior end of //ij' whereas in Synaptomys it 

 is a little longer, ending slightly behind the anterior end of m^; 

 the mammary formula is 2 — 2 = 8 in Mictomys, instead of 

 1 — 2=6 as in Synaptomys. But in certain other features 

 Mictomys is the more specialized. The strongly curved upper 



