46o MURID^— ORCADENSIS 



width across the lateral processes of the supra-occipital) ; the inter- 

 parietal pentagonal and very narrow; the inter-orbital region com- 

 paratively broad; the squamosals separated anteriorly by 1-4. mm., 

 their post-orbital crests moderate with feeble antero-external processes ; 

 the jugals with well-marked central expansions, their upper borders 

 boldly convex ; w^ each with a deep fourth outer infold. For external 

 and cranial dimensions, see tables at pp. 462-3. 



(3) M. orcadensis rousaiensis, Hinton. 



1913. M. o. ROUSAIENSIS, M. A. C. Hinton, Ann. and Mag. Nat. Hist, November 

 1913, 460 ; described from Rousay, Orkneys ; type, an adult male. No. 12.7.5.7 

 of British Museum collection. 



Distribution : — Rousay, Orkneys. 



Description : — This sub-species is externally indistinguishable from 

 0. orcadensis, but has a skiill intermediate in character between the latter 

 and o. sandayensis. 



The brain-case is short and broad, as in 0. orcadensis ; the occiput 

 occasionally depressed ; the inter-parietal and the supra-tympanic 

 fossae nearly as in 0. orcadensis ; the squamosals separated anteriorly by 

 i-o mm., their post-orbital crests and processes intermediate in 

 saliency between those oi 0. sandayensis and o. orcadensis ; the degree of 

 inter-orbital constriction is also intermediate ; the jugals are always 

 slender, as in 0. sandayensis ; m^ always has a deep fourth outer 

 infold, as in 0. orcadensis. For external and cranial dimensions, see 

 tables, pp. 462-4. 



(4) M. orcadensis westrce (Miller). 



1908. MiCROTUS SANDAYENSIS WESTR^, G. S. Miller, Ann. and Mag. Nat. Hist., 

 February, 199 ; described from Pierowall, Westray, Orkneys ; type, a sub-adult 

 male, No. 8.1.2.1 of British Museum collection ; Trouessart ; Miller {Catalogue). 



1913. MiCROTUS ORCADENSIS WESTR^, M. A. C. Hinton, Ann. and Mag. Nat. Hist., 

 November 1913, 460, 



Distribution : — Westray, Orkneys. 



Description : — The colour is similar to that of 0. sandayensis, but the 

 dark shading of the longer hairs is more noticeable, and the tips of the 

 underfur are nearer to dull ochraceous buff. The under-side is coloured 

 like that of M. 0. orcadensis, but is paler, the dusky bases of the hairs 

 taking a more prominent part in the general effect. The tail is more 

 densely sprinkled with black hairs than in 0. sandayensis. 



The skull of the sub-adult type has the brain-case short and 

 broad ; the occiput is much depressed, more than in any specimen 

 of 0. sandayensis examined ; the inter-parietal nearly as reduced as in 

 0. orcadensis ; the lateral processes of the supra-occipital very salient; 

 the supra-tympanic fossae nearly as extensive as in 0. ronaldshaiensis ; the 

 inter-orbital region relatively broad ; the squamosals not more closely 



