THE SOUTH BRITISH WATER RAT 481 



angles and two infolds, the posterior angles being sometimes poorly 

 developed. 



m^ has the posterior loop preceded by three alternating closed 

 triangles, two of which are internal ; in front of these are an outer and 

 an inner triangle, more or less confluent with each other, and opening 

 into the short rounded anterior loop ; the fourth (second outer) triangle 

 is sometimes closed, and then the tooth has an appearance recalling m^ 

 oiM. ratticeps ; there are three outer and four inner infolds, and the 

 same number of salient angles, with, in addition, a more or less 

 prominent vestige of a fourth outer and a fifth inner angle anteriorly ; 

 the latter are clearly defined in very young teeth, but disappear rapidly 

 with wear. 



The remaining cheek-teeth are essentially as in normal Microtus ; 

 m^ has three outer and two inner salient angles; in m^ the anterior 

 triangles are not completely cut off from each other by the antero- 

 external fold; in m^ the outer triangles are well developed, and the 

 postero-external fold is usually deep enough to isolate the first outer 

 from the first inner triangle. 



Status, origin, and distribution in time : — A. amphibius is most 

 nearly allied to A. sapidus of southern France and the Iberian Peninsula, 

 and not to the Skandinavian terrestris. It probably owes its distinction 

 from sapidus to isolation. It is not known as a fossil, the representative 

 of the genus at Ightham being A. abbotti, a member of the older 

 scherman group. It is evidently a recent member of the British 

 Fauna, not old enough to have reached any of the islands except 

 Anglesey and Wight. 



The sub-species are as follows : — 



THE SOUTH BRITISH WATER RAT. 



^ ARVICOLA AMPHIBIUS AMPHIBIUS (Linnsus). 



1758. Mus AMPHIBIUS (species), Carolus Linnagus, Systema Natures, x., 61, xii., 62, 



1766; described from England and based on the Mus major aquaticus of Ray ; 



Berkenhout ; Turton ; Walker ; Bingley ; Donovan. 

 1828. Arvicola AQUATICA, John Fleming, History of British Animals, 23 ; the 



specific name from Leach, Syst. Catal. Spec. Indig. Mamm. and Birds, British 



Museum, 1816, 7 (a nomen nudum); Gesner (1551) ; Merrett (1666) ; Charleton 



(l668) ; Sibbald (1684) ; and Ray (1693). 

 1835. Arvicola amphibia, Leonard Jenyns, Manual of British Vertebrate Animals, 



33 (part) ; Bell, ed. i and 2 ; MacGillivray ; Owen (part ?). 

 1842. Arvicola Americana, J. E. Gray, Ann. and Mag. Nat. Hist, 226 ; described 



from half-grown specimens supposed to have been taken in South America, and 



of which co-types are in the British Museum. 



