212 MICROTIN^ 



Fur dense, long and soft in winter, shorter and harsher in 

 summer. Colour of back usually rufous, darker and richer in 

 humid and wooded regions, lighter and yellower in open country 

 and in the north, where there is some tendency to winter whiten- 

 ing. The reddish tint usually forms a more or less well-defined 

 mantle extending from the crown of the head to the rump and 

 laterally, to a greater or less degree, encroaching upon the flanks. 

 Hairs everywhere with slaty bases. 



Skull agreeing in general form with that of other members of 

 the subfamily, but more lightly built, rounder, less angular and 

 ridged than in higher genera (e.g., Microtus). Interorbital region 

 broad. TemiDoral ridges weakly indicated, always widely 

 separated in the interorbital region. Squamosals light, showing 

 little or no tendency to encroach upon the frontals or parietals 

 anteriorly ; post-orbital crests of squamosals usually weak or 

 absent. Interparietal strajj-shaped. Bony palate terminating 

 posteriorly as a thin-edged, transverse shelf continuous between 

 the alveoli of the posterior molars {m^ — m^) ; the posterior median 

 sloping sej^tum of the palate of Microtus is absent, although 

 the post-palatal lateral jiits are present, deep, and extensive. 

 Mesopterygoid fossa wide; presphenoid relatively broad. Ec- 

 topterygoid plates low. Pterygoid fossae shallow, their floors 

 not dorsal to the ventral surface of the basisphenoid. Auditory 

 bullae well inflated, simjjle, internally without spongy tissue. 



Mandible nearly normal ; groove between molars and ascend- 

 ing ramus open posteriorly, not " pocketed " by alveolus of lower 

 incisor as in Microtus, etc. ; coronoid process relatively slender 

 and angular jjrocess relatively larger. 



Dentition. — Incisors slender. UiDper incisors without grooves, 

 terminating posteriorly between the premaxillary suture and m^. 

 Lower incisors relatively shorter than in Microtus, passing from 

 lingual to labial side of jaw between ni^ and m^, but not causing 

 any marked lingual displacement of the latter tooth, and termin- 

 ating in the base of the condylar process below the level of the 

 dental foramen. 



Cheek-teeth with tubercular caps when quite unworn, showing 

 a prismatic pattern when worn. Enamel thin in superficial, 

 thick in deeper portions of the crowns. Re-entrant folds jaartly 

 filled with cement. Pulp cavities and cement spaces closing 

 below in adults, each tooth then developing two roots, an anterior 

 and a posterior, which grow in length as the crowns are worn 

 away. In extreme old age the crowns of the teeth may be entirely 

 worn away and the abraded summits of the fangs may then appear 

 alone above the gums. 



In average specimens, with well but not excessively worn teeth, 

 the enamel pattern, in most species, is characterized by the 

 rounded form of the salient angles and the confiuency of the 

 dentinal spaces, the last-named feature being particularly notice- 

 able in lower molars. In pattern m^, m^, Wg and m^ are 



