LEMMtTS 



619 



vertical. Nasal simple, squarely truncate posteriorly at level of 

 middle of zygomatic root, scarcely or not exceeded by the very 

 slender nasal branches of premaxillaries. Incisive foramina long 

 and very narrow, extending from about 1 ■ 5 mm. behind incissors 

 to within about 1 mm. of level of molar alveoli. Palate about as 

 wide as alveolus anteriorly, about twice as wide posteriorly ; 

 lateral grooves moderately well marked ; posterior emargination 

 extending nearly to level of front of m s , its median spine blunt 

 and short, but evident ; pterygoids short, the hamulars heavy, 

 curved outward posteriorly, the mesopterygoid space extending 

 over posterior margin of palate. Mandible rather slender, but 

 with conspicuously developed ridges for muscular attachment ; 

 posterior portion with a peculiar thin and sunken aspect, due 

 partly to the absence of the incisor root from this region, and 

 partly to the high masseter ridge and the conspicuous expansion 

 outward of under surface of large angular process ; articular 

 process very slender and weak, much compressed ; coronoid 

 process broad at base, but narrowing rapidly to a delicate 

 backward-curved point, the extremity of which in uninjured 

 specimens reaches to level of base of articular facet ; dental 

 foramen near posterior margin of base of articular process, and 

 on level with rim of alveolus ; a slightly larger foramen, 1 - 5 mm. 

 below and in front of dental foramen at posterior extremity of 

 depression between outer alveolar wall and inner surface of 

 ascending portion of mandible. 



Teeth. — As compared with those of the typical voles, such as 

 the members of the genera Microtus and Arvicola, the incisors are 

 weaker and the molars heavier relatively to size of skull. The 

 molars are further peculiar in the unusual 

 width of the posterior tooth, which causes 

 the cutting surface of the tooth-row to be 

 almost parallel-sided instead of backwardly 

 tapering ; main axes of upper tooth-rows not 

 essentially parallel as in the voles, but so 

 converging anteriorly that if prolonged they 

 would meet slightly in front of incisors. 

 Upper incisors rather long and distinctly 

 curved, not projecting forward, the root pro- 

 ducing a slight swelling at front of ante- 

 orbital foramen ; cross-section of shaft ap- 

 proximately semicircular, the flattened border 

 directed inward, the anterior margin pale 

 yellow in colour, slightly wider than posterior 

 margin, the greatest transverse diameter a 

 little behind middle ; enamel covering anterior 

 fourth of inner surface and extending to slightly beyond middle of 

 outer surface ; cutting surface of tooth deeply and conspicuously 

 hollowed, its anterior margin bent sharply backward internally, 

 following course of enamel, its posterior border forming a distinct 



Lemmus lemmus. 

 Teeth. X 5. 



