ARVICOLA 727 



ears, usually tinged with odiraceous-])uff or light raw-umber, 

 though not sufficiently to produce any marked contrast with 

 surrounding parts ; chest and belly a varying mixture of 

 ochraceous-l)utf and the slate-grey under colour, the two 

 becoming more intimately blended on throat and fading to a 

 light drab grey ; feet ranging from hair-brown to ecru-drab, 

 sometimes with a blackish shade ; tail blackish throughout, the 

 underside sprinkled with greyish hairs. 



Skull. — The skull of Arvicola ampJiibius is lai'ge and massive, 

 becoming conspicuously ridged and angular in fully adult 



Fig. 150. 

 Arvicola amphibius. Nat. size. 



individuals. In general form and proportions it does not differ 

 notably from the skull of Microtus agrestis, except that the brain- 

 case tends to be shorter and wider, the dorsal profile is more 

 evenly convex, without any sudden angular bending down of 

 nasals at interlachrymal region, and the upper incisors project 

 sufficiently forward so that most of their anterior surface is 

 visible when skull is viewed from above. Brain-case somewhat 

 longer than wide, the well-developed postorbital processes 

 squaring its anterior margin and imparting the general outline 

 of a parallelogram ; lateral ridges strongly developed in adults 

 and marking ofT a conspicuous flat median surface widest at 



