THE RAASAY BANK MOUSE— MICROTUS 425 



merely in their larger size. As in the latter species, m^ has a deep 

 third inner fold, and usually a large salient fourth inner angle. 



Dimensions in millimetres: — Collector's measurements of three 

 adult but not old males: head and body, no (type), 112, 114; tail, 45, 

 48, 50; hind foot, 18, 20, 20; ear, 13, 14-5, 14. 



Skull: — (two males, Nos. 79 (type) and 81), condylo-basal length, 

 25-2, 25-4; breadth, zygomatic, 14-7, 15; least inter-orbital, 3-7, 3-8; 

 mastoid, ii-8, 11-9; occipital depth, 6-4, 66; length of nasals, 7-1, 

 7-4; of diastema, 6-7, 67; of maxillary tooth-row, 62, 6-3; of 

 mandible, i6-i, 16-4; of mandibular tooth-row, 6-i, 6-i. 



Status : — Like its near relative, E. alstoni, the Raasay Bank Mouse 

 seems to be another member of the so-called " Boreal " group of species, 

 surviving in an island, because of the immunity from competition which 

 it there enjoys. Unlike E. alstoni, but in this respect resembling 

 skomerensis and casarius, it has undergone considerable specialisation, 

 apparently to fit it for subsistence upon a coarser, and probably a 

 more exclusively vegetarian, diet. It has developed relatively large 

 cheek-teeth, requiring more powerful muscles to move the jaws. 

 The strengthening of the muscles has, in turn, caused those parts 

 of the skull and mandible to which they are attached to grow 

 stronger, and in this way the many differential characters of the skull 

 noted above appear to have been brought into existence. The muscles 

 affected by the enlargement of the teeth are the temporals, masseters, 

 and pterygoids, and every one of the cranial features described is 

 correlated with the increased development of one or other of them. 



Genus Microtus. 



This genus includes the typical Microtines, which generally 

 but not invariably differ from the members of the genus 

 Evotomys in their shorter ears and tail, and in the dull brownish 

 tints of their dorsal surface. 



They are first known from the late Pliocene of Europe, three 

 or four species having been described from the British Upper 

 Freshwater Bed of Cromer. They appear also in the early 

 Pleistocene of Greenhithe, Kent (Hinton), and in most deposits 

 of later dates, as at Grays Thurrock (M. agrestoides, Hinton, 

 Proc. Geol. Assoc, 3rd June 19 10, 493, a form characterised by 

 the presence of a fourth exterior angle in ni), and Ilford, Essex. 

 Some of the early species appear to belong to the sub-genus 

 Chionomys. In North America the genus is not known earlier 

 than the Pleistocene. 



VOL. II. 2 E 



