456 MURIDAZ—ORCADENSIS 
of the jugals are boldly convex, and 7, has a deep fourth outer 
fold. In sandayenszs 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 ovcadenszs 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and the fourth outer fold of 7, is obsolete. The modifications 
of the jugals and mz, seen in the group are correlated by Hinton 
with the differential development of the two portions of the 
temporal muscles. The sub-species vowsazensis and westre are 
intermediate between ovcadenszs and sandayensts ; rousazensis, 
on the whole, is nearer ovcadensis, while westv@ in some respects 
approaches sandayensts, 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. JZ. 0. ronaldshatensis, orcadensis, and 
rousatensis inhabit islands separated by 6 to 8 fathoms of water, 
and they agree in external appearance ; wes¢7@ and sandayenses 
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 
