MIMOMYS 475 
teeth of MZicrotine; 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. 2° 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 7m and m* have each three roots. 
M. pliocenicus 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 m* is not 
reduced. In two of these later species (7. cntermedius ; and 
M. savin, Hinton, Proc. Geol. Assoc., 3rd June 1910, 491) the 
third outer infold of #, develops as in JZ. pliocenicus, 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 JZ. piocenicus ; in addition, m and m2 
have each only two roots. Hinton concludes that these later 
forms were not directly descended from JZ. plocenicus, but 
from a similar, though slightly more primitive, animal. In 
the upper Freshwater part of the Forest Bed the more advanced 
species alone occur, JZ. plhocenicus having by that period 
become extinct. 
Besides the above species, 17. newtont 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 AZ. plocenicus ; 
both in it and in JZ. vezdz of Hinton, from the Weybourne Crag, 
the tooth-roots are formed early in life. The latter differs from 
M. plhocenicus in 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 zztermedius and savinz noticed above, there 
occurs a third form, JZ. mazorz (Hinton, of. czt.); in this the 
third outer valley of 7, 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, (7. cantianus of Hinton (Hinton and White, Proc. 
Geol. Assoc., June 1902, 414; Hinton, of. czt., 491); the cheek- 
