460 MURIDAE—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 ; 7, each with a deep fourth outer infold. For external 
and cranial dimensions, see tables at pp. 462-3. 
(3) I. orcadensis rousatensis, Hinton. 
1913. M. 0. ROUSAIENSIS, M. A. C. Hinton, Anm. 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. orcadensts, but has a skull intermediate in character between the latter 
and 0. sandayensis. 
The brain-case is short and broad, as in 0. orcadensis ; the occiput 
occasionally depressed; the inter-parietal and the supra-tympanic 
fosse nearly as in 0. orcadensis ; the squamosals separated anteriorly by 
1:0 mm., their post-orbital crests and processes intermediate in 
saliency between those of 0. sandayensis and 0. 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) MW. orcadensis westre (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. saxdayensis, 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 JZ. 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. saxdayensts. 
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 fosse nearly as extensive as in 0. ronaldshazensis ; the 
inter-orbital region relatively broad ; the squamosals not more closely 
