ARVICOLA 393 



Explanation of Fig. lOG. 



Arvicola greenii Hinton. 

 Freshwater Deposit, Ostend, near Bacton, Norfolk. 



1. Right nil (type of species; B.M. Geol. Dept., No. 15984a, Green 



Coll.). 



2. Right »ii (B.3I. Geol. Dept., No. 17627a, Green Coll.). 



Arvicola prccceptor Hinton. 

 Pleistocene ; early Middle Terrace. 



3. Right 7?i.' ; Grays Thurrock, Essex. 



4. Right TOj; Grays Thurrock. 



5. Left m^ ; Grays Thurrock. 



6. Left»fti; Grays Thurrock. 



7. Left nil ^^^ ""^2 (typs of species) and a detached left m^; Grays 



Thurrock. 



8 and 9. Left nii, two mutilated specimens; Barrington, Cambridgeshire. 



10. Left ?n J ; Grays Thurrock. 



11. Right m^i Barrington. 



12. Left m^ and TO 2; Barrington. 



1 3. Left m^ ; Grays Thurrock. 



14. Right J»^; Barrington. 



15. Right TO-; Grays Thurrock. 



16. Right to'; Grays Thurrock. 



17. Left ?M>; Grays Thurrock. 



Type. — Part of a left mandibular ramus, with the incisor, m^ 

 and m.^ in place. 



Type horizon and locality. — Early Middle Terrace deposits of 

 the Thames, at Grays Thurrock, Essex. 



Range. — Known from the Middle Terrace of the Thames at 

 Grays and Ilford, Essex, and from a deposit of similar age at 

 Barrington, Cambridgeshire. 



Characters. — Size slightly larger than A. greenii from the 

 Cromerian of Bacton. Cheek-teeth persistently growing as in 

 Arvicola, but with the enamel in adult specimens differentiated 

 as in Mimomys, thick on the convex, thinner on the concave 

 sides of the salient angles. Enamel pattern (Fig. 106, 3-17) : m^, 

 w?, ?r?2, and m^ normal ; ?)i^ with the second inner infold persistent, 

 never reduced by insulation, though sometimes shallow, with 

 three salient angles on each side, the third inner angle sometimes 

 tending to become obsolete, and with two .substantially closed 

 triangles following the anterior loop ; ??ij with a persistent, 

 though sometimes shallow, third outer fold which is never reduced 

 by insulation of its internal portion, with the third outer salient 

 angle often complicated by a more or less well-developed and 

 long persistent " prism-fold," with three substantially closed 

 triangles following the posterior loop, and with four outer and 

 five inner salient angles, of which the anterior angle on either side 

 may be weakly developed or obsolete. Skull (as indicated by a 

 specimen from Barrington) with the temporal ridges fused into a 

 median crest in the hinder part of the interorbital region. Mandible, 

 so far as is known, normal. 



Remarks. — This species is represented by a left mandibular 



V.L. D D 



