Vol. 50.] FAUNA FROM THE IGHTHAM FISSURE. 197 



Microtus ( = Arvicola) arvalis. (European Field-Vole.) — Portions 

 of nearly a dozen skulls, mostly rather smaller than those referred 

 to M. agrestis, are placed in this species. Eight of them have the 

 second cheek-tooth preserved, which shows, in each case, only 

 5 angles ; while some have the hinder cheek-tooth present and 

 showing 4 inner and 3 outer angles. Several lower-jaw rami are 

 associated with these skulls ; they are rather smaller than that 

 referred to M. ayrestis, but have the front cheek-tooth showing the 

 angles characteristic of both these species, namely, 5 inner and 

 4 outer. There are many limb-bones which may belong to this 

 species or M. ylareolus, or perhaps to M. ayrestis. M. arvalis has 

 been found fossil in the Forest Bed and in a fissure-deposit near 

 Frome. It is extinct in Britain, but is now living in Middle 

 Europe from the Atlantic to the Urals, and also in Western Siberia in 

 steppe regions. In the Alps it ascends to a height of 6000 feet. 



Microtus (=Arvicola) ayrestis. (Field-Vole.) — Portions of two 

 skulls, with the characteristic upper second cheek-tooth (6 points) 

 in place, undoubtedly represent this species ; and with these are 

 associated several lower-jaw rami, which are rather larger than 

 those referred to the last species, but have the form of tooth 

 (5 inner and 4 outer angles) characteristic of both M. ayrestis and 

 M. arvalis. The limb-bones, which on account of their length might 

 be referred to this species, are not certainly determinable, the femora 

 especially being stouter and having larger heads than in the recent 

 specimen. Microtus ayrestis has been recorded from English Caves ; 

 it is now living in Middle and Northern Europe, as well as in Britain, 

 and ranges southward as far as the Alps and Pyrenees (where it 

 extends to a height of 4000 feet), but is more plentiful in the north, 

 reaching in Scandinavia to 66° of north latitude. 



Microtus (=Arvicola) ratticeps. (Northern Vole.) PI. XI. 

 fig. 11. — Two lower-jaw rami and three of the characteristic teeth 

 are all that I am able to refer to this species ; but possibly some of 

 the portions of skulls noticed under M. yreyalis may belong here. 

 The front cheek-teeth still remaining in the jaws, as well as the 

 separate teeth, have each 5 inner and 3 outer angles, the anterior 

 prism being confluent with the fourth inner prism. This and the 

 three previously noticed species are so nearly of the same size that 

 it is probable some of the smaller limb-bones already mentioned may 

 belong to the present form. M. ratticeps has been found in Caves 

 and other Pleistocene deposits. It is not now living in Britain, 

 but occurs throughout Northern Europe, from Scandinavia to the 

 Urals, and also over a large part of Siberia. 



Microtus ( = Arvicola) yreyalis. (Siberian Vole.) PI. XI. fig. 12. — 

 Nearly forty small rami, varying somewhat in size, are referred to 

 this species on account of the structure of the anterior cheek-tooth. 

 This tooth has 5 inner and 3 outer angles, the auterior enlarged 

 prism being continuous with the fifth, but shut off from the 

 fourth inner prism. This form of tooth is much like that of 



