MICROTINiE 38s 



substances, the cheek-teeth and masticatory muscles are 

 exceptionally powerful ; the skull is massively constructed, and 

 several of its paired bones, such as the maxillae, palatines, and 

 frontals, which usually remain free in other Muridce, are fused 

 together at an early stage in the existence of the individual 

 (see Winge, Vidensk. Med. Nat. For. Kj'ob., 1881, 37-50 (1882); 

 Grbnlands Pattedyr, 1902, 358; DanmarMs Pattedyr, 51, 68-79). 



The cheek-teeth are hypsodont and often endowed with the 

 power of persistent growth. Their evolution, which is analogous 

 to that of the pikas (see above, pp. 155-158), has been worked 

 out by Hinton, to whom I am indebted for a rdsumd of his, at 

 present unpublished, results. Although the actual ancestors of 

 the MicrotincB have not yet been discovered, there can be no 

 doubt that their cheek-teeth were of a brachyodont, tubercular 

 type ; the apical or first-formed and most conservative parts of 

 these teeth in recent forms still retain such a structure, and in 

 the more primitive genera, such as Dicrostonyx, remains of three 

 longitudinal rows of tubercles are present in both upper and 

 lower series when unworn, showing that the group has descended 

 from a stock with murine rather than cricetine cheek-teeth. 

 Such brachyodont teeth were adapted for crushing and grind- 

 ing ; they have gradually been converted into a most perfect 

 apparatus for slicing and shearing. The first step in the 

 process has been the atrophy, by fusion or blending, of some of 

 the less useful primitive tubercles. The cusps have thus been 

 arranged in an inner and outer alternating series of triangular 

 form, the evolution of which can be readily appreciated by 

 examining the changes in pattern shown by the molars of 

 Cricetincs in different stages of wear (Figs. 52 and 54). 



The transition from a soft and succulent to a tough and dry 

 diet has been gradual. It has been accompanied by steadily 

 increasing wear of the crowns of the cheek-teeth, by increased 

 nutrition of these organs, and by prolongation of their period 

 of growth. Since during the earlier stages growth exceeds 

 attrition, the cheek-teeth have become hypsodont ; in later 

 stages growth and wear are equal, and this condition persists 

 until death in the higher genera of the group, as in Microtus ; 

 in lower forms, as Evotomys, a time comes when the enamel 

 organs fail, and thereafter growth of the crowns ceases, the 



