EVOTOMYS 



629 



of the weakened appearance characteristic of Lemmus and 

 Myopus, its posterior border fading gradually into surface of 

 tooth or marked off by a slight angle. Lower incisor noticeably 

 more slender than upper tooth, its root extending beneath that 

 of m s , but not rising to level of cutting surface of molars, its 

 shaft nearly semicircular in cross section, the flattened surface 

 directed inward, the enamel extending over anterior half of 

 curved surface ; anterior surface of both upper and lower incisors 

 yellowish brown, posterior surface whitish. Molars relatively 

 small and weak, at first rootless and growing from a pulp as in 

 Lemmus and Microtus, the prisms extending uninterruptedly to 

 base. At a slightly later stage the extreme base is seen to be 

 smooth, forming a ring below prisms ; this ring then contracts 

 at middle and divides into two ; as the cutting surface of the 

 crown wears away and the tooth is pushed upward in the 

 alveolus, each of these secondary rings narrows and lengthens 

 into a prong or root standing in axis of tooth-row, the anterior 



FIG. 125. 

 Evotomys glareolue. Enamel pattern in two individuals. 



prong of each tooth somewhat larger than the posterior ; with 

 advancing age the prongs become relatively longer and the 

 crowns lower, so that in the final stage the crowns may com- 

 pletely wear away, leaving in their place the flattened heads of 

 the six roots. It follows that the enamel pattern, instead of 

 remaining essentially constant throughout life, as in the genera 

 with rootless molars, undergoes marked changes in the later 

 stages of crown-wear, gradually losing its definiteness and finally 

 disappearing. As compared with that of European voles of 

 other genera the enamel pattern is characterized by a general 

 lack of sharp angularity, especially in the salient angles on 

 inner side of the maxillary teeth, where the loops assume the 

 form of circles or semicircles rather than that of closed triangles, 

 and by the arrangement of the prisms of the mandibular teeth 

 in imperfectly alternating pairs, so that completely isolated and 

 closed triangles are the exception rather than the rule. In 

 crown length m 1 slightly exceeds to 3 , which in turn is usually 

 somewhat longer than irr, the discrepancy in neither case very 



