394 MICROTIN^ 



ramus, containing the incisor, m^ and m^, five detached examples 

 of the nil ^^'i fo^"^ detached specimens of m^, together with many 

 other teeth and fragmentary limb bones collected by Mr. Kennard 

 and the writer long ago at Grays ; by a series of remains including 

 seven examples of the ?»j and four of the m^, collected at Grays 

 by Mr. G. White; by a small series, including specimens of 

 the m^ and 7n^ together with the frontal bones of an old adult 

 and other fragments of the skull, from the brickearth at Barring- 

 ton, Cambridgeshire, collected by Mr. Kennard and the writer. 



The characters of the molars show very clearly that this Middle 

 Terrace Arvicola is a direct descendant of the Late Pliocene 

 Mimomys, a genus known to have lingered to the times represented 

 by the High Terrace of the Thames. The Middle Terrace Arvicola 

 is further shown by the peculiarities of its m^ (persistence of the 

 third outer valley and frequent persistence of the " prism-fold ") 

 to be closely related to, possibly identical with, A. greenii, the 

 species occurring in the Late Cromerian beds near Bacton, Norfolk, 

 and like A. greenii it is to be regarded as a direct descendant of 

 M. majori of the Cromerian beds at West Runton. As previously 

 explained the High Terrace Mimomys appears to be a survivor 

 of the same stock. It is noteworthy and suggestive that although 

 three species of Mimomys occur at West Runton, descendants of 

 only two of them are found at Bacton, and in the Middle Terrace 

 one alone has been found. It is further suggestive that the primi- 

 tive stock thus shown to have survived to Middle Terrace times 

 is precisely the one which agrees with modern Arvicola in the 

 method by which the third outer fold of the m-^ is reduced. 

 The Barrington frontlet is important, showing firstly that 

 we have old adidt material before us, so that there is good 

 evidence for the rootlessness of the teeth, and secondly that A. 

 prceceptor had made a great advance from the condition of the Late 

 Pliocene Mimomys, and perhaps also from that of A. greenii, by 

 acquiring temporal muscles which were as powerfully developed 

 anteriorly as they are in modern species of Arvicola. 



4. jAirvicola mosbachensis Schmidtgen. 



1910. Hypudceus amphibius von Reichenau, Notizblatt d. Vereins f. 

 Erdkunde, Darmstadt, [4], 31, p. 122. 



1911. Microtus mosbachensis Schmidtgen, Notizblatt d. Vereins f. 

 Erdkunde, Darmstadt, [4], 32, p. 186. 



Co-types. — Fragmentary jaws and teeth in the Museums of 

 Mainz, Prankfurt-am-Main, and Wiesbaden. 



Type horizon and locality. — Mosbach Sands, Mosbach ; Lower 

 Pleistocene. 



Characters. — Size small ; cheek-tooth row 6-7 mm. measured 

 on grinding surface. Enamel pattern essentially as in Arvicola 

 or later species of Mimomys ; dentinal spaces narrowly con- 

 fluent. Cheek-teeth apparently persistently growing. 



