468 MURID^— LOCALLY EXTINCT VOLES 



Ightham probably marks the date of the arrival of the species 



in Britain, since no arva/zs-like " vole " is represented among the 



many microtine fossils recovered from the Crayford brickearth, 



a deposit one stage older than that of Ightham. 



Microtus anglicus (Hinton, Ann. and Mag. Nat. Hist, July 



1 910, 36, first referred by Nehring to a Httle-known existing 



species, M. gregalis, Pallas) is a very remarkable species 



belonging to the highly specialised Asiatic group which 



Kastchenko calls Stenocranius. The 



skull is very long and narrow, with 



the long inter-orbital region greatly 



constricted and bearing a sharp median 



crest ; the median septum of the hinder 



palate is long and thin. The upper 



cheek-teeth are similar to those of 

 Fig. 75.— (i) Fossil Right OTj OF , . ^\- c i- 



M. anglicus of the late Pleistocene M. aVVallS. m^ (Fig. 75) IS of peculiar 



of England ; (2) of Mkrotus ratti- form ; it has the posterior loop and five 



cebs (K. and B.) : Pleistocene of , . . , , ^ n/r- a 



England; 8 times life size. closed triangles of normal Microtus, 



but the fourth outer angle (counting 

 from behind) is reduced, and the fourth outer fold obsolete ; the 

 outer border of the anterior loop is long and straight, and the 

 tooth has only three outer salient angles. »% has the third outer 

 angle obsolete. The Stenocranius group is regarded by Hinton 

 as an offshoot of the arvalis group, its cranial and dental 

 peculiarities being apparently results of an increased develop- 

 ment of the temporal muscles ; this speciaHsation is similar in 

 kind, though more intense in degree, to that described in the 

 orcadensis group at p. 453. It is thus possible that instead of 

 there being any specially close affinity between M. anglicus 

 and the similarly modified forms of central Asia, the former 

 may afford an instance of parallel evolution from a common 

 arvaloid stock. 



M. anglicus first appears in Britain in, and is perhaps the 

 most characteristic element of, the late pleistocene fauna 

 (Ightham), and it, or closely allied forms, had a wide range in 

 western and central Europe. So far as is known, all such forms 

 are now extinct in this region. 



Microtus ratticeps (Keyserling and Blasius, Mem. Acad. Imp. 

 Sci. Nat., St Petersburg, iv., 3, 333, 1841 (1845) ; described from 



