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 JZ. arvalzs. These 
are readily distinguished by their 7°, 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: J/. orcadensis, inhabiting Pomona and the South 
Orkneys, and JZ. sandayensis (with two sub-species, viz., 
M. s. sandayensis from Sanday, and JZ. s. westre, 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—JZ/. 
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 JZ. orcadenszs must be associated JZ. sarnius’ of 
Guernsey, Channel Islands, and the extinct late pleistocene 
M. cornerz,* 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 
1 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, zdz@., July 1910, 35. 
