THE ST KILDA FIELD MOUSE 



S4I 



normal coloration, for Mr Eagle Clarke has found {op. cit.), from a long 

 series of specimens taken in the months of September and October, that 

 in about two-thirds of the adults, " and many of the immature ones, 

 the throat, chest, and abdomen are white, and only washed with brown 

 along the narrow median ventral line. As a result the demarcation 

 between the peppery reddish-brown upper-, and the pale under-surface, 

 is pronounced in most examples, and renders the species very similar 

 to " A. sylvaticus. " About one-third of the adults and the majority of the 

 younger specimens have the under-surface more or less strongly washed 

 with buff." The dark dorsal line is well developed. In young 

 individuals the backs are greyer, the bellies more rufous than in adults 

 (see Steele Elliott's description of the type specimen. Zoologist, 1895, 

 426). In the skull the smooth and rounded brain-case is rather 

 depressed ; the post-molar length is not shortened as is usual in 

 hebridensis, though the bullae are rather small ; and the nasals are 

 relatively long (see proportional measurements in the table at p. 538). 

 The cheek-teeth are as in sylvaticus. 

 Bimeusions : — 



1 Cheek-teeth only slightly worn. 



Remarks ; — Specimens with head and body between 100 and 1 1 3 mm. 

 are probably adolescent ; full-grown (which in most Muridce means 

 "senile") specimens are those with the head and body near 120 mm. 



For cranial dimensions, see table at p. 538 above. 



Status: — A. hirtensis is a well-marked member of the sylvaticus 

 group. The latter has been established in Britain since the late 

 Pliocene (Forest Bed), a fact which goes far to support the view that 

 this species is truly indigenous upon St Kilda. The characters of 

 A. hirtensis are probably to be regarded as the results of insular 

 specialisation. For remarks upon the position of hirtensis within the 

 sylvaticus group, see above under Genus. 



Habits : — Mr Eagle Clark says that this mouse is most abundant 

 where coarse grass prevails, although it is to be found almost every- 

 where, as in the crofted area, the neighbourhood of houses, on the faces 

 of the cliffs, and on the sides and tops of the hills. It finds congenial 

 retreats in the rough stone-built " cleits," and in the walls surrounding 

 VOL. II. 2 M 2 



