356 MICROTIN^ 



20. M. majori. Right ?»i adult, crown view. U.F.B., West Runton 



(Savin Coll., No. 824.2). 



21. M. cantianits Hinton. Right Wj adult, imperfect, type of species, 



crown view (outer and inner views, PI. XIII, figs. 11, llo). 

 High Terrace of the Thames, near Greenhithe, Kent {0. White 

 Coll.). 

 22, 22o. M. plioccenicus. Right rn^ adult, crown and inner views. Wey- 

 bourne Crag, East Runton {A. S. Kennard). 

 23. M. neu'toni. Right m' adult, crown view. Upper Pliocene, 

 Tegelen-sur-Meuse {B.M., presented by Mr. and Mrs. Clement 

 Reid). 

 24-26. M. intermedius. Right and two left m^, adult, crown views. 

 U.F.B., West Runton (Mas. Pract. GeoL, Nos. 906^, B, and C). 



27. M. newtoni. Right m^ imperfect, adult, crown view. " Clay- 



gravel," East Runton {3Ius. Pract. GeoL, No. 979j5). 



28. Mimomys sp. Right m^ imperfect, crown view ; with two short 



roots and no trace of enamel islet. N.C., Thorpe {Noru'ich 

 Mus., No. 971h (1), Filch Coll.). 



three salient angles on each side, its second inner infold usually 

 persistent, but in one species {M. flioccenicus) subject to reduction 

 by insulation; yn^ with three closed triangles following the 

 posterior loop as in Arvicola, and an anterior loop of complex 

 structure. In some species the third outer infold of this tooth 

 persists throughout the crown (newtoni group) ; in others this 

 fold is reduced by the insulation of its internal part. In the 

 earlier or more primitive species the enamel islet, which results 

 from the reduction of this outer fold, persists as a conspicuous 

 feature of the grinding surface until an advanced stage of wear 

 has been reached ; but in later or more progressive forms the 

 islet is an ephemeral feature which vanishes long before the 

 molar roots begin to develop. Third outer angle of m.^ in some 

 species cleft by a peculiar fold called the " prism-fold," ^ which 

 may or may not persist throughout the crown. 



In the earlier, or more brachyodont, species the lower incisor 

 passes from the lingual to the labial side of the jaw beneath 

 the posterior root of m^ ; but in later, or more hypsodont, species 

 the incisor crosses between mg and m^, so that mg like m^ is 

 wholly labial to the incisor shaft. 



Skull imperfectly known. Temporal ridges in some species, 

 at all events, widely separated in the interorbital region. Masse- 

 teric plate (outer wall of infraorbital canal) essentially as in modern 

 voles. Palate of normal Arvicoline type with postero-lateral 

 bridges and pits, and with the postero-median sloping septum 

 clearly defined although short and broad. Mandible essentially 

 as in most other voles ; the lower incisor ascending the condyloid 

 process to a point often well above the dental foramen ; the 

 termination of the alveolus of the incisor often producing a well- 

 marked hump on the outer surface of the jaw. 



Relationships. — The relationships of Mimomys have been 

 fully discussed above. It has no doubt the same ancestry as 

 ' For the significance of this structure see p. 111. 



