484 MURID^— ARVICOLA 



individuals occur almost as generally as the brown. Macpherson and 

 Aplin regarded these black animals as merely varieties. In 1910 

 Miller {pp. cit.) confirmed MacGillivray's original opinion, but on 

 different grounds. He showed that the black colour of Scottish 

 individuals, being a geographically limited character accompanied by 

 smaller size, is of sub-specific value. The sub-species undoubtedly needs 

 further investigation, especially in regard to the black colony inhabiting 

 the fen country, which may be composed only of melanistic examples 

 of true amphibius. 



Distribution : — North of the watersheds of the Clyde and Tay, the 

 Black Water Rat is generally present and abounds in suitable streams 

 and lochs, apparently to the exclusion oi A. a. amphibius. There are 

 no records as to how far it goes up the hills. Some writers {e.g. 

 Lydekker) report it as absent from Argyll, but this is not the case. 

 In the Forth district a colony has been known to exist near Colinsburgh, 

 Fife, for many years (W. Evans). On the other hand, records of it 

 from the Orkneys (Baikie and Heddle, addendum) are probably based 

 on confusion with Microtus orcadensis ; those from Islay, Mull (E. A. 

 Alston), and Skye (Macpherson and Aplin, op. cit.), on M. agrestis. 

 This sub-species must not be confounded with sporadic instances of 

 melanism occurring in England. In Cambridgeshire and Norfolk, 

 however, black individuals predominate so as almost to suggest that 

 they represent an isolated colony of M. a. reta. 



Description : — This Water Rat is on the average smaller than the 

 typical sub-species. The hind foot usually reaches a length of not more 

 than 30-32, and the condylo-basal length of the skull usually less than 

 42 mm. The colour is normally darker, and black often replaces brown 

 on the upper surface ; melanistic specimens are frequent. 



Black and brown individuals may occur in the same litter (W. Evans, 

 Ann. Scott. Nat. Hist., 1910, 53). 



Skull (Adult Male, British Museum, 5.5.12.1): — Condylo-basal 

 length, 41-9; breadth: at zygomata, 24; at inter-orbital constriction, 

 4-2; at occiput, 18-5 ; median occipital depth, 11-3 ; length: of nasals, 

 12-2; of diastema, 13-5; of mandible, 27-9; maxillary tooth-row, 10-4; 

 of mandibular tooth-row, 10-3. 



Status : — Judging by its distribution and analogy with other 

 mammals, the Black is an older form than the Common Water Rat, 

 by which it has been driven out from the lower parts of the country, 

 except perhaps Cambridge and Norfolk. But it cannot be a very 

 old member of our fauna, since it is absent from all the Scottish 

 islands. A statement of Boyd Watt — that the black form was first 

 recognised in the Clyde area in August 1842 at Ballantrae, Ayrshire 

 (whence John Thompson Sinclaire sent it to Thompson, see Nat. Hist., 

 Ireland, iv., 1856, 13), but by John Colquhoun's time it had become 



