724 EODENTIA 



1896. Arvicola Miller, North Amer. Fauna, no. 12, p. 66, July 23, 1896 



(sub-genus). 

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



February, 1908 (genus). 



Type species.- — Mus amphibius Linnseus. 



Geographical 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. 



Bemarhs. — Though not sharply differentiated from Microtus, 

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

 genus. Its range does not extend beyond the Palsearctic 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 OF ARVICOLA. 



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



and m 2 forming evident protuberances on lower 



surface of mandible in old individuals ; habits 



strictly aquatic (amphibius group). 

 Nasals at widest region conspicuously narrower 



than rostrum (Great Britain) A. amphibius, p. 725. 



Condylobasal length of skull usually 42 to 

 44 mm. ; colour dark, but melanism infre- 

 quent (England) A. a. amphibius, p. 730. 



Condylobasal length of skull usually 40 to 

 42 mm. ; colour very dark, melanism frequent 



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



Nasals at widest region nearly as wide as rostrum 



(Spain and southern France) A. 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 



(Pyrenees 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 



m-, and ra 2 never forming protuberances on lower 



surface of mandible ; habits aquatic or terres- 

 trial (terrestris group). 



