MIMOMYS 357 



Dolomys and Apistoinys, and from it, in turn, the modern genera 

 Arvicola and Phaioinys seem to be descended. 



Species. — Eight species are at present recognized, but no doubt 

 others will have to be defined when more satisfactory material 

 is obtained. Of the eight, seven are known from British deposits. 

 Recent work, done with the advantage of much more exten- 

 sive material, confirms Forsyth Major's conclusions as to the 

 distribution in time of the chief British species. From the 

 Norwich Crag only the older forms, M. plioccBuicus and its allies 

 and perhaps M. newtoni are known ; whereas in the Upper Fresh- 

 water Bed at West Runton we find only the newer species 

 M. inter )nedius, M. savini and M. majori. But in the East 

 Runton deposit ('' shelly crag ") of intermediate age both the 

 older and the newer species occur together. One species, M. 

 cantianus, probably a descendant of M. majori, occurs in the 

 early Pleistocene High Terrace Drift of the Thames. 



As already indicated, the species fall into two groups; in 

 one, in this respect the more primitive, comprising M. newtoni, 

 M. majori and M. caidianus, the third outer infold of the jn^ 

 is persistent; but in the other, comprising M. plioccenicus, 

 M. reidi, M. savini, M. intermedins and M. pusiUus this infold 

 is reduced by insulation, and in the later members of the group is 

 almost completely obliterated in all but the very youngest stages 

 of wear. 



In each group the molar roots are developed earlier in the 

 older species than in the more modern ones, and the vanishing 

 elements of the crown become in successive geological horizons 

 more and more ephemeral. Thus the general trend of dental 

 progress seen within this genus, as elsewhere in the subfamily, 

 is towards the persistent growth (complete hypsodonty) of the 

 molars and the simplification of their crown structure. 



1. fMimomys pliocaenicus Forsyth Major. 

 1872. Arvicola sp. Forsyth Major, Atti Sec. Ital. Sci. Nat., 15, p. 389. 

 1874. Arvicola amphibius Blackmore and Alston, P.Z.S., 1874, p. 462, 



fig. Iffl (in part); Tuccimei, Mem. d. Pontificia Accad. d. Nuovi 



Lincei, 9, 1897, p. 35. 

 1882. Arvicola (Evotomys) infcrmedius Newton, Vert. Forest Bed, 



p. 83, pi. xiii, fig. 12 (in part). 

 1889. Arvicola pliocenica Forsyth Major, in Weithofer, Jahrb. d. k.k. 



geolog. Reichsanstalt, 39, p. 86. 

 1902. Mimomys pliocwnicus Forsyth Major, P.Z.S., 1902, 1, p. 103, 



fig. 8 (text-fig. 13); Hinton, Proc. Geol. Assoc, 21, 1910, p. 491; 



Mehely, Ann. Mus. Nat. Hungar., 12, 1914, p. 186. 

 1907. Microtus {Mimomys) pliocxBnicus Neviton, Bull. Soc. Beige 



Geo]., 21, Mem., p. 592; Riitten, Die diluviale Saugethiere der 



Nicderland, 1910, p. 88. 



Type. — Florence Museum. Part of a left mandibular ramus, 

 with the incisor, m^ and mo in place; figured by Forsyth Major, 

 P.Z.S., 1902, 1, p. 103, figs" 8 and 9 text-fig. 13). 



