Vohs of the orcadensis Group. 101 



M, corner!, Hititon, is a large vole whose remains occur in 

 the late Pleistocene deposits of England and possibly France 

 (Neschers, Bruniquel, &c). It is clearly a forerunner of 

 M. sarnius and M. orcadensis; in fact, the three together may 

 conveniently be separated from the other arvaloid voles as 

 the orcadensis group. M. corneri is distinguished generally 

 from its two allies by its less expanded zygomatic arches, 

 shorter nasals and molars, longer diastema, and slightly 

 straighter and more protruding incisors. The nasals are 

 narrowly rounded behind and not cleft ; the median septum 

 of the palate has a well-marked ventral groove; the molars 

 are of the normal arvalis pattern, but very light. The 

 temporal muscle was evidently much weaker, as a whole, than 

 in orcadensis. The ridges, though clearly defined in the 

 adult, are not salient. Posteriorly they are relatively wider 

 apart even than in M. sarnius, and the interparietal retains 

 to the full the form seen in young orcadensis and in the 

 adults of less modified voles ; the lateral processes of the 

 supraoccipital are as salient, however, as in M. o. ronaldshai- 

 ensis or o. westroz, and the supratympanic fossse are therefore 

 considerably more extensive than in sarnius, though slightly 

 less so than in sandayensis. The jugal has a boldly convex 

 upper border, and its central expansion is therefore maiked. 

 The interorbital region is in the degree of its constriction 

 intermediate between those of orcadensis and sandayensis, 

 and the temporal ridges fuse into a low but sharp inter- 

 orbital crest ; the anterior ends of the squamo-als approach 

 each other to nearly the same extent as in orcadensis. On 

 the other hand, the posterior frontal process remains broad 

 and semicircular throughout life as in the youngest stages of 

 orcadensis (<;/'. dimension b); the postorbital crests of the 

 squamosal, though extensive, are but slightly salient and 

 without any antero-external projection. In these weak 

 crests, as well as in the long narrow form of the brain-ease, 

 there is some similarity to the skull of M. o. ronaldshaiensis 

 as well as to that of M. sarnius. The occiput agrees with 

 that of the latter in relative depth. 



It is difficult to decide which is the more primitive, 

 M. sarnius or M. corneri ; both are clearly more primitive 

 than M. orcadensis, and M. corneri occupies a somewhat 

 central position between the two living species. The short 

 nasals, small teeth, narrow zygomata, and, above all, the 

 entirely unmodified interparietal and coronal suture of the 

 fossil lead me to regard it as the most primitive of the three, 

 although it has advanced further than M. sarnius in the 

 approximation of the anteiior parte of the squamosals. 



