456 Mr. M. A. C. Hinton on the 



ensis ; the occiput in the subadult skull and in that of a small 

 but old female is as depressed as in o. sandayensis, but in the 

 adult males it is as lofty as in o. orcadensis. The anterior 

 portion of the temporal muscle is apparently more strongly 

 developed than in o. orcadensis, less so than in o. sandayensis ; 

 and its effect upon the characters of the interorbital region and 

 anterior part of the brain-case is accordingly an intermediate 

 one. In the small female skull the interparietal is as greatly 

 reduced as in o. orcadensis, and the supratympanic fossae are 

 noticeably more extensive because of the greater saliency of 

 the lateral processes of the supraoccipital ; in the adult males 

 the interparietal is slightly less modified and the supra- 

 tympanic fossae slightly smaller than in o. orcadensis. The 

 posterior portion of the temporal muscle is apparently, there- 

 fore, more nearly like that of o, orcadensis than that of o. san- 

 dayensis, but has a tendency to weaken. In its external 

 appearance the Rousay vole is quite like o. orcadensis. 



More material is required before one can be quite certain of 

 the cranial characters of o. ivestrm. The type is a young adult 

 (condylo-basal length 27*2 mm.) ; its skull and teeth agree quite 

 closely with Miller's figures 138 a and a' 139, which, how- 

 ever, represent o. sandayensis ; the depression of the occiput 

 is more marked than in any of the skulls from Sanday seen by 

 me : ^71 has the fourth outer fold obsolete, although in other 

 specimens it is well developed; the jugal is slender ; the 

 interorbital region is rather wide and the squamosals not 

 more closely approximated anteriorly than in o. orcadensis, but 

 in older skulls probably some further progress will be seen in 

 this region. Posteriorly the temporal ridges approach con- 

 siderably more than in o. sandayensis and only slightly less 

 than in the Rousay form ; the interparietal is accordingly 

 modified to nearly the same extent as in o. orcadensis ; the 

 width across the lateral processes of the supraoccipital is 

 relatively very great, and the supratympanic fossae are there- 

 fore far more extensive than in o. sandayensis, and, in fact, 

 considerably exceed those of o. orcadensis. Like those of the 

 Rousay form, its cranial and dental characters appear to be, 

 on the whole, intermediate between those of o. orcadensis and 

 o. sandayensis ; in addition, as described by M.ller, its external 

 appearance is of an intermediate character also. 



The vole of South Ronaldshay is represented by three 

 specimens before me; only one skull is fully adult and 

 sufficiently perfect for detailed investigation. In external 

 appearance, the form of ^r\, and the conspicuous expansion 

 of the jugal this form agrees with o. orcadensis. The inter- 

 parietal is even more modified than in the latter form ; the 



