Voles of the orcadensis Group. 457 



lateral processes of the supraoccipital are considerably more 

 salient and the supratympanic Eossse attain their maximum 

 extension in the group ; the posterior portion of the temporal 

 muscle therefore appears to be somewhat more strongly 

 developed than in o. orcadensis. On the other hand, the 

 squamosals are more widely separated anteriorly than in any 

 other Orcadian form, although the posterior process of the 

 frontals is as much narrowed as in o. sandayensis ; the post- 

 orbital crest of the squamosal is relatively feebly developed, 

 and the brain-case, as a whole, appears to be longer, narrower, 

 and more lofty than in o. orcadensis-, the anterior portion 

 of the temporal muscle is therefore apparently weaker than 

 in o. orcadensis. The nasals, diastema, and molars agree in 

 relative lengths with those of o. sandayensis rather than with 

 o. orcadensis. 



It would appear from the facts given above that when the 

 posterior portion of the temporal muscle is the predominant 

 one the jugal is heavy and ^1 complex; this is the con- 

 dition found in the {South Ronaldshay vole and o. orcadensis. 

 When the anterior portion of the muscle tends to become 

 the predominant one, the jugal becomes light and ^n tends 

 to be simplified by losing the fourth outer fold ; this con- 

 dition is seen fully developed in o. sandayensis. Many facts, 

 which cannot be discussed here, but of which some have 

 already been dealt with long ago by Winge, lead me to 

 believe that in this group predominance of the posterior 

 portion of the temporal muscle is a more primitive condition 

 than is predominance of the anterior portion. The Orcadian 

 voles may therefore be arranged as subspecies of M. orcadensis 

 in the following order, beginning with the least modified : — 



M. orcadensis, Millais. 

 (1) M, orcadensis ronaldshaiensis, subsp. n. 



Hab. South Ronaldshay. 



Type. An adult male (B.M. 7. 11. 16. 1). 



Diagnosis. — External appearance as in M. o. orcadensis. (Collector's 

 measurements of type : head and body L26mm. ; tail 11 ; hind foot 19; 

 ear L1'6.) Skull distinguished from that of o. orcadensis by its longer 

 and narrower brain-case ; the squamosals slightly more widely separated 

 anteriorly, their postorbital crests Less salient, and with no antero-extemal 

 projection : interparietal Blightly more reduced, Bupratympanic fosses 

 much more extensive laterally; jugal and .-t. t as in o. orcadensis. 

 (Cranial measurements: condylo-basal length -!> mm.; zygomatic 

 breadth L7'6; interorbital constriction 3 - 8 ; occipital breadth ]_'.»: 

 occipital depth 1- ; nasals 9*2; diastema v 7 : maxillary cheek-teeth 

 (alveolar) 6*7 ; mandible 18*6; mandibular cheek-teeth (alveolar) 6*7.) 



Ann. & Mag. X. Hist. Ser. 8. Vol. sii. 33 



