ON THE MAMMALS OF FAIR ISLE 67 



sandy, with no distinct line of demarcation." It is confined 

 to the islands of Lewis and Barra in the Outer Hebrides. 



4. Mus sylvaticus hirtensis, Barrett-Hamilton. This sub-species 



is very like M. s. hebridensis, but can be distinguished by 

 its " slightly larger size and greater amount of buff or 

 yellowish-brown colour on the under-side." Only known 

 from the island of St. Kilda. 



5. Mus sylvaticus wintoni, Barrett-Hamilton. This is a large, 



well-developed Field-mouse, much brighter in coloration, 

 both on the upper and under sides, than M. s. intermedius. 

 At all ages there is a distinct breast-band across the chest, 

 with a " longitudinal stripe in the centre extending forward 

 about 5 mm. and back along the sternum about 10 mm." 

 This mouse is found in small colonies scattered over 

 England, chiefly in the south, and in the western parts of 

 Europe. 



In form and proportions the Fair Isle mice more closely 

 resemble M. s. wintoni than either M. s. hebridensis or M. s. 

 Jiirtensis, but differ from that form in having a smaller ear, 

 in which respect they approach hebridensis : the tail is con- 

 siderably longer than in either hebridensis or Jiirtensis, and 

 the hind-foot is smaller but quite as stout as in both these 

 forms. The head and body measurements of the two largest 

 examples in the series, male and female, are 121 mm. and 

 115 mm. respectively, as compared with 112 mm. in hebri- 

 densis, 110 in Jiirtensis, and 115 in wintoni, the measure- 

 ments of the largest examples given in the above paper. 



In coloration the Fair Isle form is richer than M. s. 

 intermedius, but not so bright as wintoni, and without the 

 yellow breast-band characteristic of the latter sub-species. 

 The line of demarcation is very distinct, and the under-side 

 is white, showing no trace of the buff found in hebridensis 

 and Jiirtensis. 



Captain Barrett-Hamilton has shown * that among the 

 results produced by isolation is a shortening of the ear and 

 lengthening of the hind-foot, but in the Fair Isle mice, 

 though the shortening of the ear is noticeable, it is not 

 accompanied with an increase in size of the hind-foot, as is 

 the case in the mice from the Hebrides and St. Kilda, while 

 there is no tendency to become buff on the under-side like 



1 "P.z.s." 1900, p. 395. 



