352 MICROTIN^ 



Acrorhiza, in which the posterior root of j/ij li^s partly on 

 the inner and partly on the outer side of the incisor. 



Pleurorhiza, in which the posterior root of m^ is wholly 

 labial to the incisor. 



To Acrorhiza Mehely ascribes the genera Dolomys, Mimomys 

 (as he restricts it), and his genera " Pliomys " and Apistomys. 

 In Pleurorhiza he includes " Fiber " (= Ondatra), Phenacomys, 

 Evotomys, and his genus " Microtomys." The Acrorhiza com- 

 prise the most ancient and generalized forms ; the Pleurorhiza 

 the more modern and progressive types. According to Mehely 

 the Pleurorhiza must have evolved from the Acrorhiza by a 

 change in the kind and manner of mastication ; the postero- 

 internal angle of m^ must have gradually ceased to have a 

 functional importance, and therefore the part of the root lying 

 immediately below it atrophied and eventually disappeared. 

 He says that the older form of mastication was a grinding 

 (" mahlende ") process, and that this was replaced by a per- 

 cussive (" stossende ") action. Since grinding serves for the 

 detrition of harder vegetable substances, and percussion for the 

 treatment of a softer diet, Mehely infers that the Acrorhiza 

 lived under dry steppe-like conditions ; whenever a damper 

 " forest-period " followed, softer vegetation came in vogue and 

 " percussive " voles were evolved. In the most primitive forms, 

 m- and m^ had three roots each ; later on, the number was 

 reduced to two roots to each tooth. Mehely thinks that the 

 purpose of the third root was to attach the tooth as firmly as 

 possible and that its presence, therefore, is evidence of the harder 

 and tougher food of the most ancient forms as compared with 

 that of their, in this respect, degenerate descendants. 



In my opinion this classification together with the ingenious 

 theories and most of the new genera to which it has given origin 

 are all based upon a series of misconceptions. I can confirm 

 Mehely in his observation that the posterior root of m^ in M. 

 plioccenicus rests straddle-wise upon the dorsal surface of the 

 lower incisor, whereas in M. intermedius the molar root in question 

 passes wholly to the labial side of the incisor. This difference, 

 however, is due solely to the circumstance that whereas M. plio- 

 ccenicus remained a relatively brachyodont animal, M. intertnedius, 

 like all other modernized " Fibrince," has made comparatively 

 great strides towards hjrpsodonty. The lower molars by deepen- 

 ing their crowns have had their bases brought into closer relations 

 with the shaft of the incisor, which found its way through 

 the entire horizontal ramus of Microti at a very early stage in 

 the history of the group, at a time long before either the crowns 

 or the roots of the molars had penetrated the ramus to such 

 deep levels; a structure so well established and so important 

 as this perforating incisor shaft would not give way before the 

 gradual advance of the growing bases of the cheek-teeth and the 



