190 MICROTIN^ 



molar series; but in Lemmus, with still heavier teeth, the 

 diastema is considerably longer than the tooth-row. Anterior 

 palatal foramina rather small; somewhat shorter and much 

 narrower than in Synaptomys and Myopus. Cheek-tooth rows 

 rapidly divergent posteriorly. Palate posteriorly essentially as 

 in Microtus, but the posterior median sloping septum is repre- 

 sented merely by a short, broad and free spinous process, the 

 inner margins of the post-palatal pits effecting a junction with 

 the base, instead of with the tip of the median process, relatively 

 far forwards and under the cover of the lateral bridges. Meso- 

 pterygoid fossa very short and broad. Hamular processes of 

 pterygoids short, thickened, and closely applied to the eustachian 

 portions of the auditory bullte. Pterygoid fossse unusually 

 short and deep ; their floors conspicuously dorsal to the ventral 

 surface of the basisphenoid. Basisphenoid and fore-part of 

 basioccipital relatively broad. Auditory bullae ovate or pyri- 

 form, their inner borders very slightly convex ; mastoid portion 

 and tegmen tympani noticeably though slightly inflated ; external 

 meatus produced as a very short but rather weU-marked tube; 

 internal structure of bulla essentially as in Synaptomys and 

 Myopus, but the spongy tissue of its walls denser than in either. 



Mandible with very stout horizontal rami, in correlation 

 with the heavy cheek-teeth and wholly lingual courses of the 

 lower incisors, and hght and delicate ascending rami ; condylar 

 processes relatively long and slender; coronoid processes short 

 and delicate ; angidar processes robust, the lower border of 

 each thickened, splayed out distally as a conspicuous facet for 

 the insertion of the anterior portion of the masseter lateralis 

 muscle. Alveolus of lower incisor terminates on the inner 

 surface of the jaw near the front edge of m,^; alveolar capsules 

 of TOj, 1)12 ^^^1 "*3 make slight but usually noticeable protuber- 

 ances just below or on the sharply salient crista masseterica. 



Dentition. — Incisors rather deeper than broad, their enamel 

 pale yellow in colour. Upper incisor with anterior face sharply 

 bent into a wider outer and a narrower inner reflected portion ; 

 outer part of anterior face often with a vestigial trace of the 

 anterior longitudinal groove ; worn surface of the tooth peciiliar, 

 tubular or gouge-like, the soft dentine wearing away for a con- 

 siderable distance above the cutting edge of the enamel ; alveolar 

 capsule terminating just above the palatal surface of the maxilla 

 slightly in front of m^. Lower incisor slender, extending back- 

 wards as far as m^. 



Cheek-teeth rootless, very broad and heavy ; enamel about 

 equally thick on convex and concave sides of salient angles, 

 becoming thin at points of re-entrant angles, and very thin or 

 altogether lacking at tips of salient angles; re-entrant folds 

 partly filled with cement. Enamel pattern (Fig. 77) characterized 

 by the inequality of the outer and inner salient angles and 

 infolds; in upper teeth (except in the hinder part of m^) 



