Voles of the orcadensis Group. 455 



of the temporal fascia, imparting to the bone its central 

 expansion, and that the anterior lower cheek-tooth ^71 has a 

 deep fourth outer fold. 



The youngest skulls of o. sandayensis examined are slightly 

 older than the majority of the young skulls from Pomona; 

 making due allowance for this, there is little difference 

 between them. The nasals and diastema are already slightly 

 longer, the molars slightly shorter, and the occiput is a little 

 more depressed than in the young of 0. orcadensis. In the 

 next stage (subadult, condylo-basal length 26*J:-27'6 mm.) 

 nasals and molars have acquired their adult proportions, the 

 former a little longer, the latter a little shorter, than in adult 

 0. orcadensis ; the diastema is now as long as in the latter, and 

 becomes in adults slightly longer; the occiput is now con- 

 siderably lower, and becomes still more depressed in theadults. 

 With regard to the parts influenced by the anterior portion of 

 the temporal muscle, the interorbital region becomes progres- 

 sively more constricted with age, and in adults is considerably 

 narrower than in o. orcadensis ; the anterior portions of the 

 squamosals approximate so closely in adults that only the 

 sharp interorbital crest separates them ; each postorbital 

 (squamosal) crest develops a more salient antero-external 

 projection ; and the narrowing of the posterior process of the 

 frontals and coronal emargination of the parietals is still 

 more marked than in 0. orcadensis. It is evident from these 

 features that the anterior portion of the temporal muscle is 

 relatively stronger than in the latter form (<f. dimensions 

 a, b, and 3 of the table). On the other hand, the figures 

 show (dimensions 8, 9, and 9-8) that in the transition from 

 youth to age the posterior part of the temporal muscle 

 develops much less than in 0. orcadensis', the difference between 

 young and old skulls is here less striking, and the form of 

 the interparietal is considerably less modified. We recall 

 that in the Sanday vole the upper border of the jug a] is but 

 slightly convex, that the bone is therefore slender, and that 

 the fourth outer fold of ^o is reduced and often obsolete. 

 The condylo-basal length of adult skulls ranges from 27*5 to 

 30" 1 mm. ; they are therefore as large, if not slightly larger 

 than, those of old o. orcadensis. 



The youngest skull from Kousay is subadult (condylo-basal 

 length 27*1 mm., the temporal ridges still faint and widely sepa- 

 rated in the interorbital region). On comparing it with the 

 adults from the same island, a similar general progress is seen 

 to take place with growth to that traced in 0. orcadensis and 

 0. sandayensis. The ,.,. t ,\ has always a deep fourth < uter Bold 

 as in 0. orcadensis ; thejugal is always slender as in 0. sanday- 



