MTMOMYS 353 



latter have therefore had to mould and adjust themselves to the 

 presence of the incisor. In all modern voles the bases of m^ 

 and wig pass to the labial side, and that of ni^ passes to the 

 lingual side of the incisor ; this difference in the courses of the 

 deep portions of the three cheek-teeth is occasioned by the 

 spiral twisting of the incisor as it passes from before backwards. 

 M. plioccFnicus represents the penultimate stage of the process 

 of adjustment; both roots of w^, the anterior root of Wj, and 

 both the roots of m.^ are already fully adjusted to the presence 

 of the incisor; the posterior root of mg alone remains partly 

 dorsal to the incisor shaft. 



Mehely's notion of the course of evolution followed by the 

 Microtine dentition is beyond question a complete inversion. 

 Persistently growing molars of prismatic form, invested by 

 well-differentiated enamel sheets, are characteristic of many 

 other rodents besides voles and lemmings ; in all cases they 

 have been evolved gradually from brachyodont tubercular teeth 

 as the rodent groups possessing them have become addicted to 

 a harder and coarser vegetable diet. In such rodents the molar 

 dentition has ceased to be a grinding or bruising apparatus ; it 

 has become a more or less powerful shearing and slicing machine. 

 With the deepening of the teeth, their roots become shorter 

 and develop later and later in the history of the individual; 

 finally they are not formed at all. The teeth are implanted in 

 the jaws by their crowns, which acquire peculiar curvatures; 

 by this curvature and their continual growth the teeth are kept 

 tightly " keyed " together by contact with each other near 

 the wearing surface ; in addition, cement is developed in the 

 most highly specialized forms and, partly filling the re-entrant 

 folds of the teeth, gives attachment to a special series of ligaments 

 which bind the teeth to the alveolar walls. Having regard to 

 the work done by the machine it may be said that such a mode 

 of implantation affords an incomparably firmer union of the 

 teeth with the jaws than any that could be secured by means 

 of fangs alone. 



Range. — The genus is abundantly represented in the later 

 Pliocene deposits of S.E. England, and is known from deposits 

 of similar age in Holland, Belgium, Italy and Hungary. It is 

 also known from the early Pleistocene of England and Hungary. 



Characters. — Cheek-teeth developing roots in the adults 

 in earlier species or in old age in the later species; each tooth 

 with two roots, except ni^ and m^, which in the earlier forms 

 possess three. Cement usually present in the re-entrant folds, 

 but in some species lacking. Enamel usually differentiated into 

 thick and thin tracts, the thicker portions forming the convex 

 sides of the sahent angles instead of their concave sides as in 

 most living voles. Enamel pattern very similar to that of 

 Arvicola in general appearance ; m^, m^, m^ and m.^ of normal 

 pattern; m^ rather simple, nearly as in Arvicola, usually with 



