NOTES ON TWO COLLECTIONS OF MICE 223 



On arranging these specimens in order of individual age, 

 as judged by the degree of wear shown by the teeth, it seems 

 clear that the earlier adult coats are characterised by colder, 

 less rufous dorsal tints than are the later phases. The three 

 young adults from the Butt of Lewis, described above, have 

 backs which match that of No. 13 from Barvas closely. 

 Nos. 2 and 12 (Barvas) show much black on their backs. 

 The specimens with much worn teeth have bright red backs, 

 and form a very uniform series. The reddening of the 

 back appears to be a gradual process transpiring between 

 adolescence and old age. As regards the ventral coloration, 

 there appears to be a corresponding gradual change with 

 advancing age. Thus the young adults have silvery bellies 

 with the buff suffusion absent, or else represented merely by 

 a small throat spot : the suffusion becomes conspicuous only 

 in the aged specimens, and in these it varies from a more or 

 less well-developed median stripe along the chest and belly 

 to a more or less general wash over all the underparts. 



We are not able to discover any sexual differences of 

 coloration in this collection. It would appear, however, 

 that the aged female skull is distinctly smaller than the 

 aged male skull : the condylo-basal length in such females 

 varies between 23-2 and 24 mm., and in such males between 

 24-2 and 25 mm. ; the range found by one of us on a former 

 occasion was 22-7 to 24-2 for five females, as against 23-5 to 

 24-7 for five males from Lewis. 



The young specimens resemble those from the Butt of 

 Lewis, and call for no further comment. 



In an earlier paper ^ it was pointed out that skulls from 

 eastern Lewis (Stornoway) are slightly smaller than those of 

 typical A. h. hebrtdensis from Uig and neighbouring portions 

 of western Lewis, and that apart from size some differences 

 of proportion exist between the two series. In order to 

 have any value, comparisons of this kind must be based 

 upon skulls of equal age. Unfortunately, none of the 

 specimens from the Butt of Lewis is old enough to be used 

 in this way. Seven of those from Barvas are of use, and 



^ Hinton, op. cit., 125 ; and British Mammals, ii., 533 and 539. 



