ORCADENSIS 455 



are greatly enlarged (Fig. 72 and dimensions 8, 9, and 9-8 of 

 table at p. 462). 



The pleistocene M. corneri occupies a somewhat central 

 position between M. sarnius and M. orcadensis, but it is on 

 the whole, as might have been expected, more primitive than 

 either. The incisors are straightened 'and protruding, the 

 cheek-teeth of normal arvalis pattern, but light and short ; the 

 short nasals terminate roundly behind, the diastema is long, the 

 posterior palatal septum grooved. The zygomata are narrow, 

 the jugals heavy with boldly convex upper borders. The 

 brain-case is long and narrow, the occiput intermediate in depth 

 between 0. orcadensis and 0. sandayensis, the temporal ridges 

 fuse into a low but sharp inter-orbital crest, the post-orbital 

 crest of the squamosals is weak, though long, the inter-parietal, 

 coronal suture and posterior frontal processes are entirely 

 unmodified in the adult, retaining to the full the form seen in 

 young orcadensis ; anteriorly, however, the squamosals encroach 

 upon the frontals, the distance between the right and left bones 

 being no greater in adults than in adult 0. orcadensis. 



The Guernsey M. sarnius has an external appearance which 

 recalls M. agrestis. In its cranium it is more specialised than 

 M. corneri, except only in one respect, the wide anterior 

 separation of the squamosals. The rather short nasals are 

 cleft behind ; the median septum of the hinder palate has no 

 ventral groove. 



M. orcadensis stands considerably higher than either M. 

 corneri or M. sarnius. Owing to the richness of colour and 

 length of the fur, the external appearance of its less modified 

 southern sub-species is not very different from that of some of 

 the Hebridean forms of agrestis ; in its northern races, ivestrce 

 and sandayensis, the colours tend to become pallid. The 

 temporal muscles are throughout considerably stronger than in 

 either of its allies, and in the transition from youth to age exert 

 a much more powerful influence upon the form of the skull. 

 In ronaldshaiensis and orcadensis the posterior portions of the 

 muscles appear to be more powerful than the anterior ; 

 accordingly in these forms the modification of the inter-parietal 

 IS more profound (in adults) than elsewhere. The post-orbital 

 crests are relatively weak, the occiput deep. The upper borders 



