MIMOMYS 475 



teeth of Micro tines ; in later stages (Fig. 78) this infold is 

 reduced, its inner part being converted into an enamel islet 

 which persists as a conspicuous feature of the grinding surface 

 until a very advanced stage of wear has been reached, m^ of 

 this species has its second inner infold reduced by insulation 

 of the internal part in a similar way. The roots of the cheek- 

 teeth are developed early in the existence of the individual, 

 and nj}- and m^ have each three roots. 



M. plioccBiiicus also occurs in the lower Forest Bed series at 

 East Runton, and is there associated with more highly developed 

 species, in which, however, the second inner infold of n^ is not 

 reduced. In two of these later species i^M. intermedins ; and 

 M. savini, Hinton, Proc. Geol. Assoc, 3rd June 1910, 491) the 

 third outer infold of m-y develops as in M. plioccsnicus, but the 

 process of reduction is accelerated ; the enamel islet is found 

 only in young stages of wear, and entirely vanishes before the 

 roots begin to appear, which is at a later moment in the life of 

 the individual than in M. plioccsnicus ; in addition, w^ and m^ 

 have each only two roots. Hinton concludes that these later 

 forms were not directly descended from M. plioccsnicus, but 

 from a similar, though slightly more primitive, animal. In 

 the upper Freshwater part of the Forest Bed the more advanced 

 species alone occur, M. plioccsnicus having by that period 

 become extinct. 



Besides the above species, M. newtoni of the Norwich Crag 

 and the lower part of the Forest Bed, is a small form with 

 cheek-teeth of a less reduced type than those of M. plioccsnicus ; 

 both in it and in M. reidi of Hinton, from the Weybourne Crag, 

 the tooth-roots are formed early in life. The latter dififers from 

 M. plioccsnicus \xi its small size, confluent dentinal spaces, and 

 the much more transitory presence of the islet in m-^. In the 

 upper Freshwater Bed of West Runton (top of the Forest Bed 

 series), in addition to intermedius and savini noticed above, there 

 occurs a third form, M. maj'ori {Hinton, op. cit.); in this the 

 third outer valley of m-i is not reduced at all, but is normally 

 developed. In the early pleistocene High Terrace Drift of 

 the Thames Valley remains of another species have been 

 detected, M. cantianus of Hinton (Hinton and White, Proc. 

 Geol Assoc, June 1902, 414 ; Hinton, op. cit., 491) ; the cheek- 



