458 MURID^— ORCADENSIS 



he obtained no specimens until August 1887, and none were preserved 

 until September 1 898 ; his description of orcadensis 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 arvalis group of continental 

 Europe ; nt^ lacking the small third inner angle of the agrestis group ; 

 7«j, except in the most modified sub-species, having a deep fourth outer 

 fold ; and m^ 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 M. agrestis 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 M. 

 arvalis, the skull differs in its greater size and zygomatic breadth ; 

 relatively smaller bullae ; 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) M. orcadensis ronaldshaiensis, Hinton. 



1913. MiCROTUS ORCADENSIS RONALDSHAIENSIS, M. A. C. Hinton, Ann. 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 M. 0. 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 



