724 



RODENTIA 



1896. Arvicola Miller, North Amer. Fauna, no. 12, p. GG, July 23, 1896 



(sub-genus). 

 1908. Arvicola Miller, Ann. and :Mag. Nat. Hist. 8th ser., i., p. 195, 



February, 1908 (genus). 



Ty2^e species. — Mus ampMbius Linnpeus. 



Oeographical distribution. — Palsearctic region from Great 

 Britain eastward into Asia, and from the Mediterranean coast 

 north to northern Scotland and the shores of the Arctic Ocean. 



Characters. — Like Microtus, but plantar tubercles only five 

 (a very rudimentary sixth rarely present) ; third upper molar as 

 in Chionomys ; external form modified for aquatic or subterranean 

 life. 



Bemarks. — Though not sharply diflerentiated from Microtus, 

 this group is so natural that it may conveniently be treated as a 

 genus. Its range does not extend beyond the Pala^arctic region. 

 About fifteen forms are known, eleven of which occur in western 

 Europe. Some of these are strictly aquatic in their mode of life, 

 others fossorial, while in two (^A. terrestris and A. scherman 

 scherman) the habits appear to vary locally or with season. 



KEY TO THE EUROPEAN FORMS OP ARVICOLA. 



Size large, hind foot more than .30mm.,condylobasal 

 length of fully adult skulls usually more than 

 40 mm. ; skull not fossorial in form, the rostrum 

 and occiput tending to be squarely truncate, the 

 upper incisors slightly projecting ; roots of m^ 

 and nu forming evident protuberances on lower 

 surface of mandible in old individuals ; habits 

 strictly aquatic {avipliihius group). 

 Nasals at widest region conspicuously narrower 



than rostrum (Great Britain) A. ampliibius,j). 725. 



Condylobasal length of skull usually 42 to 

 44 mm. ; colour dark, but melanism infre- 

 quent (England) A. a. avijjhihius, -p. 130. 



Condylobasal length of skull usually 40 to 

 42 mm. ; colourvery dark, melanism frequent 



(Scotland) A. a. reta, p. 732. 



Nasals at widest region nearly as wide as rostrum 



(Spain and southern France) ^4. sapidus, p. 732. 



General colour rather light, the sides and face 

 clear yellowish brown (Spain, except As- 



turias ; lowlands of southern France) A. s. sapidus, p. 733. 



General colour dark (as in amphibius), the sides 

 and face strongly " lined " with black 



(Pj'reuecs and Asturias) A. s. tenebricus, p. 735. 



Size medium or small, hind foot usually less than 

 30 mm., condylobasal length of fully adult 

 skulls usually less than 40 mm. ; skull slightly 

 or conspicuously fossorial in form, the rostrum 

 and occiput tending to be obliquely truncate, the 

 upper incisors noticeably projecting ; roots of 

 ?», and w„ never forming protuberances on lower 

 surface of mandible ; habits aquatic or terres- 

 trial (terrestris group). 



