ORCADENSIS 453 



Group ORCADENSIS. 



Characters and status :— The Grass Mice of the Orkneys ' 

 belong to a peculiar group which appears to be an offshoot of 

 a form not known in Britain, the continental M. arvalis. These 

 are readily distinguished by their n^, which differs from that of 

 M. agrestis in lacking the small third or posterior inner angle, 

 so that it possesses only four dentine spaces. They are all 

 comparatively large animals with a tendency to develop 

 exceptionally strong temporal muscles resulting in prominent 

 modifications of the skull. 



Miller regards these mice as belonging to two species, 

 namely : M. orcadensis, inhabiting Pomona and the South 

 Orkneys, and M. sandayensis (with two sub-species, viz., 

 M. s. sandayensis from Sanday, and Af. s. westrcs, from Westray), 

 living in the North Orkneys. Further specimens, including 

 many old individuals, recently taken by Ogilvie-Grant on 

 Sanday and Rousay, enabled Hinton (whose work was done 

 since the key on pp. 377-381 was published) to show that 

 Miller's arrangement cannot be sustained, and that all the 

 Orkney Grass Mice must be referred to a single species — M. 

 orcadensis. It appears that, as a result of long segregation in 

 small islands, in this respect affording a complete parallel with 

 the agrestis group in the Hebrides, the Orkney mice have 

 become differentiated into at least five closely allied sub-species. 



With M. orcadensis must be associated M. sarnius'^ of 

 Guernsey, Channel Islands, and the extinct late pleistocene 

 M. corneri,^ the remains of which occur at Ightham, Kent, 

 and possibly also in France ; whether any continental forms 

 have similar affinities remains for future work to decide. 



Hinton has shown that the cranial modifications of the 

 orcadensis group are dependent upon the strength and size of 

 the temporal muscles. In the young these muscles are feeble. 

 Consequently young skulls of all members of the group are 

 similar to each other, and have a brain-case resembling that of 



' Millais searched the Shetlands for " voles," but, although such animals seemed 

 to be known to the inhabitants, no specimens were forthcoming. 

 " Miller, Ann. and Mag. Nat. Hist., May 1909, 420. 

 ^ Hinton, ibid., July 1910, 35. 



