504 MURID^— APODEMUS 



Mag. Nat. Hist., January 1907, 121), however, show a similar 

 combination of characters, perhaps indicating relationship with 

 Apodemus, but for Eurasian mammals the genus is a useful 

 and practical institution. 



Although not apparently capable of wide variation from the 

 generic type, these mice show, nevertheless, great plasticity 

 before the moulding influences of their environment. Consider- 

 ing the wide range of the genus, the number of its species is 

 relatively few ; but most of these species are differentiated 

 into numerous sub-species, which are to be regarded as purely 

 local developments or adaptations. The characters which 

 betray this plasticity are principally those of the pelage, 

 coloration, and the proportions of the peripheral organs ; the 

 size and form of the skull, and, to a lesser degree, of the teeth 

 are also subject to local modification. 



In the A. sylvaticus group, to which all the British forms belong, the 

 fur is typically soft, the darker hairs of the back are diffusely distributed 

 and mingled with lighter ones, and the general colour of the upper 

 parts is russet or tawny in more or less .sharp contrast with the light 

 belly. The ears, tail, and limbs, particularly the feet, are relatively 

 long. There are six mammse, the anterior pectoral pair being absent. 

 The skull is always without supra-orbital beads. The cheek-teeth have 

 the outer and inner rows of tubercles relatively well developed ; cusp i 

 is present in both m^ and m^ ; cusp 7 is well developed in m^ ; the outer 

 " accessories " of m-^ are rather large and include vestiges of cusps 6, 7, 

 and n. The British representatives of this group are : — 



A. sylvaticus, Britain, Ireland, and many of the smaller islands 



(including Skye and Bute). 

 A. hebridensis, Hebrides (exclusive of Skye and Bute). 

 A. hirtensis, St Kilda. 



A.fridariensis, Fair Isle and Mid Yell, Shetlands. 

 A.flavicollis, South Britain. 



Of these forms A. sylvaticus shows a tendency to split up into 

 several local races or sub-species within the British area. A. hebridensis, 

 hirtensis, 3lX\A fridariensis may be regarded as local developments from 

 the primitive sylvaticus stock of the region, which appear to have 

 resulted from the segregation of this stock upon small islands; the 

 characters which distinguish them from the parent stock are slight, and 

 all three might well be regarded as mere sub-species of sylvaticus were 

 it not for the fact that hebridensis and fridariensis have in turn under- 

 gone a further differentiation into a number of quite recognisable and 



