696 



RODENTIA 



enamel pattern this species is so readily distinguishable by its 

 large size and dark colour that it needs no special comparisons. 

 The nearest known relatives, apart from M. sandayensis, are 

 Mierotus sarnius of Guernsey and the extinct Microtus corner i 

 Hinton,* from the Late Pleistocene deposits of the Ightham 

 Fissure, Kent, and Langwith Cave, Derbyshire. 



6, 1 al. Eousay Island, Orkneys. W. Cowan (c & p). 4. S. 22. 1-2. 



6. Sandwick, Pomona J. G. Millais (c & p). 4.6.21.1. 



Island. {Type of species.) 



6(5.5 9. Loch Stenues, Pomona N. B. Kinnear (c & p). \ n' -i r,' o' i~ a' 



6, 2 9. S. Ronaldshay Island. Edinburgh Museum (p). 7. 11. 16. 1-3. 

 6, 9. Orkneys. ■ J. G. MiUais (c & p). 5. 11. 22. 1-2. 



MICROTUS SANDAYENSIS Millais. 



(Synonymy under subspecies.) 



Geographical disirihutioii. — North Orkney Islands, Scotland. 



Diagnosis. — Like Microtus orcadensis but size slightly less 

 (hind foot, 17-19 mm. ; condylobasal length of largest skulls 

 about 27 '5 mm.), and colour not so dark, the back approaching 

 hair-brown rather than mummy-brown ; skull with brain-case 

 noticeably flattened, auditory bullae somewhat reduced in size, 

 and zygoma showing little if any tendency to become expanded 

 at middle. 



Bemarhs. — For detailed descriptions see accounts of the two 

 subspecies. 



The voles of the North Orkney Islands are distinguished 

 specifically from Microtus orcadensis of the southern group by 

 the peculiar flattened form of the brain-case. The degrees of 

 distinctness of the Orkney voles among themselves appear to 

 bear a direct relation to the depth of water separating the 

 islands, and therefore presumably to the length of time that the 

 different colonies have been isolated. The numerous specimens 

 of M. orcadensis examined from four islands of the southern 

 group show no indication of the existence of local forms ; the 

 depth of the channels separating these islands from each other 

 ranges from six to eight fathoms. Between the islands inhabited 

 by this species and those occupied by M. sandayensis lies a narrow 

 but comparatively deep strait, with seventeen to twenty fathoms 

 of water. Finally between Sanday and Westray with their 

 different though not completely segregated forms, the depth of 

 the water is intermediate, ten to twelve fathoms. 



* Ann. and Mag. Nat. Hist., 8th ser., vi, p. 35, July, 1910. The 

 characters by which the fossil is distinguished from Microtus sandayensis 

 are less apparent than those separating the two living Orkney species. 



