458 MURIDA:—ORCADENSIS 
he obtained no specimens until August 1887, and none were preserved 
until September 1898; his description of orvcadenszs did not appear 
until July 1904. 
Description :—This is a large mouse with the hind foot 17 to 20, 
and the condylo-basal length of the skull in adults, 27-5 to 30 mm. 
The southern races are dark coloured, the northern relatively pallid. 
The cheek-teeth are essentially as in the arvalzs group of continental 
Europe; #2” lacking the small third inner angle of the agrestzs group; 
m,, except in the most modified sub-species, having a deep fourth outer 
fold ; and 7, the third outer angle distinct but small. In addition to 
the, in some forms, strikingly narrow, subquadrate or pentagonal 
inter-parietal, and the other characters correlated with the exceptionally 
powerful temporal muscles (discussed above on p. 453), the skull 
differs from that of JZ. agresézs in its slightly greater breadth (zygomatic 
breadth equals 60-61 per cent. of condylo-basal length); relatively 
longer and anteriorly more gradually expanded nasals ; slightly longer 
diastema ; shorter and lighter cheek-teeth ; and smaller, less rounded 
and more angular auditory bulla. The nasals are cleft behind by a 
small frontal process, and the posterior palatal septum bears a well- 
marked ventral groove (Fig. 73, D). From that of the continental J7/. 
arvalts, the skull differs in its greater size and zygomatic breadth; 
relatively smaller bulla ; narrower palate; and lighter cheek-teeth ; in 
addition, adults are remarkable for the far greater perfection of that 
cranial specialisation which results from the increased strength of the 
temporal muscles. 
Moults take place twice a year, at the end of May and in August 
and September. They may be gradual, the new hairs replacing the 
old as they drop out; but sometimes the long black hairs and the 
light tips of the shorter hairs of the back are shed in a mass, leaving 
the animal with only the dusky bases of its fur in a condition which 
has been mistaken for melanism. 
Further special characters are noted under the sub-species which 
are as follows, commencing with the most primitive and ending with 
the most modified :— 
(1) MW. orcadensis vonaldshaiensis, Hinton. 
1913. MICROTUS ORCADENSIS RONALDSHAIENSIS, M. A. C. Hinton, Amn. and Mag. 
Nat. Hist., November 1913, 457; described from South Ronaldshay, Orkneys ; 
type, an adult male, No. 7.11.16.1 of British Museum collection. 
Distribution :—South Ronaldshay, Orkneys. 
Description :—This sub-species cannot be distinguished externally 
from MV. o. orcadensis, but differs in the following cranial characters :— 
The brain-case is longer and narrower; the squamosals are slightly 
more widely separated anteriorly, their post-orbital crests less salient 
