THE RAASAY BANK MOUSE—MICROTUS 425 
merely in their larger size. As in the latter species, #* 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, r1o (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, 11-8, II-9; occipital depth, 6-4, 6-6; length of nasals, 7-1, 
7-4; of diastema, 6-7, 6-7; of maxillary tooth-row, 6-2, 6-3; of 
mandible, 16-1, 16-4; of mandibular tooth-row, 6-1, 6-1. 
Status :—Like its near relative, . a/stonz, 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 £. adstonz, but in this respect resembling 
skomerensis and ce@sarius, 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 (JZ. agrestocdes, Hinton, 
Proc. Geol. Assoc., 3rd June 1910, 493, a form characterised by 
the presence of a fourth exterior angle in 7*), 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. 
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