42 MICROTIN^ 



the molars as they have become hypsodont have invaded deeper 

 levels of the jaw, and there they have had to mould themselves 

 and adjust their curvatures to the shaft of the incisor already 

 present. In the lemmings it is otherwise; the molars have 

 become hypsodont much sooner, probably before the growing 

 base of the incisor had pushed back to the level of TO2. 



Among the voles Evotomys is one of the most primitive 

 genera, although its species show a fairly wide range in the 

 degrees of specialization which they have severally obtained. In 

 this genus, and in the closely related but more progressive genera 

 discussed below, the skull is characterized by the structure of the 

 bony palate, which terminates behind as a simple, thin-edged, 

 transverse shelf, not provided with a postero-median sloping 

 septum ; deep postero-lateral pits are, however, present, but they 

 pass forwards freely under the edge of the shelf. The palate 

 of Evotomys is thus very similar to that of Dicrostonyx and 

 Lemmus, and strikingly dissimilar in appearance from that of 

 Synaptomys and the higher voles such as Arvicola and Microtus. 

 The auditory bullae are always well inflated, thin walled, and 

 simple, and in nearly all species they lack all trace of internal 

 spongy tissue. The cheek-teeth are of limited growth, develop- 

 ing two roots each in adult stages of wear ; in pattern they are 

 more or less simplified or reduced longitudinally; cement is 

 present in their re-entrant folds. In outward form the species 

 show few signs of specialization in any particular direction. 



In the most primitive species (members of the glareolus group) 

 the skull is rather lightly and delicately built, with the temporal 

 ridges weakly developed and widely separated in the interorbital 

 region, and with the post-orbital crests of the squamosals small 

 though distinct. The cheek-teeth are small and the incisors 

 slender ; the lower incisor passes from the lingual to the labial side 

 of the jaw as in all voles, but its passage between mg and m^ takes 

 place at such a low level in relation to m^ that this tooth is not 

 noticeably displaced. The dentinal spaces of the molars are 

 characteristically confluent, semi-opposed and not perfectly 

 alternating as in the higher voles, and the salient angles, in adult 

 stages of wear, are peculiarly rounded ; m^, to^, TOj, and m^ are 

 reduced as in normal species of Microtus ; m^ in its least-reduced 

 form presents five salient angles on each side ; m^ is typically 

 reduced to a pattern somewhat resembling that seen in the 

 OTj of such voles as Microtus nivalis, and consists of a posterior 

 loop, five triangles, and a short anterior loop. This pattern is an 

 old one in the genus, dating from at least the Upper Pliocene, 

 and in normal members of the glareolus group young examples of 

 the nil show no trace of ephemeral complications in the anterior 

 loop ; but in some far-eastern forms traces of a fourth outer fold, 

 which is reduced by insulation of its internal part, may be seen. 

 At the other end of the series, though apparently connected 

 with the more primitive forms by a long series of gentle grada- 



