ON THE VOLE AND SHREW OF THE ORKNEY ISLANDS 



ences mi^ht be met with in the examination of several skulls 



o 



from the same species. Undoubtedly variations do occur 

 among animals of the same species, but they are not so 

 marked as those shown above. And, further, if there is 

 variation in one or several points, there is agreement in most 

 features. 



An analysis of the indices may be briefly summarised as 

 follows : 



The breadth of the cranium (compared with the length) 

 of M. orcadensis is approximately equal to that of M. 

 amphibius, but markedly greater than that of M. agrestis 

 and E. glareolus. 



The oblique height of the cranium is about the same in 

 M. orcadensis, amphibins, and agrestis. In E. glareolus it is 

 decidedly less. 



When the vertical height of the cranium of M. orcadensis 

 is compared with that of the others, it is found to be not 

 very different from that of E. glareolus, but less than in 

 M. amphibius, and greater than in M. agrestis. 



A very striking feature in the skull of M. orcadensis is 

 the degree of development of the temporal fossa, and, par- 

 ticularly, the close approach it makes to the middle line 

 in its more anterior part. This is shown by the Stephanie 

 index, which is much smaller in this animal than in any of 

 the others. 



Another very obvious feature in the cranium is the much 

 greater sagittal diameter of the interparietal bone. This is 

 very clearly demonstrated if the bone be measured in two 

 directions transverse and sagittal and an index com- 

 puted ; the transverse diameter being taken as i oo. 



It is clear that the sagittal diameter of the interparietal 

 bone of M. orcadensis is more than half the transverse ; 

 whereas in the others it is less than half. 



