4S6 MURID^E— ORCADENSIS 



of the jugals are boldly convex, and m-i has a deep fourth outer 

 fold. In sandayensis the anterior portions of the muscles are 

 predominant, and therefore the changes in the fore part of the 

 brain-case attain their culmination. On the other hand, the 

 posterior portions are weaker than in orcadensis and the inter- 

 parietal is less reduced. The occiput is depressed ; the jugal 

 convexities have disappeared so that these bones are slender ; 

 and the fourth outer fold of m-^ is obsolete. The modifications 

 of the jugals and Wj seen in the group are correlated by Hinton 

 with the differential development of the two portions of the 

 temporal muscles. The sub-species rousaiensis and westrce are 

 intermediate between orcadensis and sandayensis ; rousaiensis, 

 on the whole, is nearer orcadensis, while westrce in some respects 

 approaches sandayensis, as will be appreciated from the descrip- 

 tions on pp. 458-61. 



Miller points out that the distinctness of the Orkney Grass 

 Mice amongst themselves appears to bear a direct relation to 

 the depth of the channels between the islands, and therefore 

 presumably to the length of time during which the colonies 

 have been isolated. M. 0. ronaldshaiensis, orcadensis, and 

 rousaiensis inhabit islands separated by 6 to 8 fathoms of water, 

 and they agree in external appearance ; westrce and sandayensis 

 with their more pallid coloration are cut off from the three 

 southern forms by 17 to 20 fathoms, and are separated from 

 each other by a 10 to 12 fathom strait. 



Origin:— The orcadensis group appears to have arrived in 

 south-eastern England in late pleistocene times, probably from 

 France by way of the Channel Islands, which explains its 

 absence from Skandinavia. It spread northward through 

 Britain, becoming specialised as it dispersed, and eventually 

 reached the Orkney district which was then part of the main- 

 land ; later, on the severance of the Orkneys from the mainland, 

 portions, probably the northern first, became detached as 

 separate islands. In these segregation has played its part, 

 and differentiation of sub-species has been the result. At the 

 time the group reached Scotland the Hebrides were apparently 

 already separated from the mainland, for we have no evidence 

 of the presence upon them at any time of a Grass Mouse of 

 the present type. Hinton suggests to me that on the mainland 



