200 Jill. G. S. Miller on 



approaching bister, the arrangement of colour as in M. san- 

 dayensis sandayensis, but light tips to hairs of underfur 

 more neaily dull ochraceous-buff, and dark shading from 

 longer hairs more noticeable ; underparts light ochraceous- 

 buff, nearly as in M. orcadensis, but colour not so rich, and 

 clouding due to slaty bases of hairs more evident ; feet and 

 tail as in true satidayensis, but sprinkling of dark hairs on 

 upper side of tail more conspicuous). 



Measurements. — Type. Head and body 108 mm. ; tail 34 ; 

 hind foot 18 ; ear from meatus 10-5. Skull : condylo-basal 

 length 27'2; zygomatic breadth 16-2; interorbital con- 

 striction 3-6 ; occipital breadth 12 ; occipital depth (median) 

 6'4; nasal 7'8; diastema 8'2; mandible 17*4; maxillary 

 tooth-row 6'6 ; mandibular tooth-row 6*8. 



Specimens exannned. — Seven, all from Westray Island, 

 kindly placed at my disposal by ]\lr. Kinnear. 



Remarks. — The voles of the North Orkney Islands differ 

 from those of the southern group in their smaller size, lighter 

 colour, and in the peculiar low, flattened form of the brain- 

 case. In typical Microtus sandayenst's the first lower molar 

 invariably (12 specimens) differs from that of M. orcadensis 

 (53 specimens) in the shallowness of the anterior outer re- 

 entrant angle, which is much less developed than that of the 

 inner side (in M. orcadensis, as in the continental members 

 of the arvalis group, the two angles are approximately equal). 

 Four specimens of M. s. westrce have tiiis aberrant type of 

 tooth, while in two the angles are of normal depth. The 

 colour is less pallid than in the typical form, though not so 

 dark as in M. orcadensis. 



The degrees of distinctness of tlie 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. 

 Specimens of M. orcadensis showing no indication of the 

 existence of local forms on the various islands have been 

 examined from Rousay, Pomona, Shapinshay, and South 

 Ronaldshay, the channelsseparating which range from 6 to 8 

 fathoms in depth. Between the islands inhabited by this 

 species and those occupied by M. sandayensis lies a narrow 

 but comparatively deep strait, with 17 to 20 fathoms of water. 

 Finally, between Sanday and Westray, with their different, 

 though not completely segregated forms, the depth of the 

 water is intermediate, 10 to 12 fathoms. 



