532 MURID^— APODEMUS 



that of sylvaticus, having the brain-case smoothly rounded ; in hamiltoni, 

 however, the fore part of the brain-case is angular and distinctly ridged. 

 In all forms it is distinguished from sylvaticus by its shorter post-molar 

 length and smaller bullae, and usually by its greater palatal length, 

 longer diastema, and tooth-row. These differences will be appreciated 

 best from an examination of the tables on pp. 518 and 538. 



The external and cranial dimensions are given in the tables at 

 pp. 536-9. Further special characters are noted under the sub-species, 

 which are as follows : — 



{i) A. hebridensis hebridensis, de Winton. 



For Sjmonymy, see under species. 



Distribution : — Lewis, Outer Hebrides. 



Description : — The typical Hebridean Field Mouse differs from A. s. 

 sylvaticus in its larger size, stouter build, longer feet, shorter ears, and 

 dull under side. In general size it, when old, about equals A.f. wintoni, 

 but its ears are smaller than those of the much smaller sylvaticus. A 

 pectoral spot is present and usually rather longer than in sylvaticus, but 

 not forming a collar. In colour the 1back and flanks are about as in 

 sylvaticus. The typical adults from Uig, western Lewis, collected in 

 September, have the under-parts heavily washed with buff, and show no 

 distinct line of demarcation ; the tail is uniformly brownish-grey, and 

 is shorter and thicker relatively than in sylvaticus. Attention was 

 called by de Winton (Barrett-Hamilton, Proc. Zool. Soc, Londpn, 1900, 

 404) to the possible occurrence of sexual differences in the pelage of 

 this form ; he noticed that "' young males appear to lose the dull hues 

 of immaturity of the upper side, and to don the reddish colour of 

 maturity at an earlier age than do the females, so that young males and 

 females of the same age are actually distinguishable by their colour 

 alone." In a series of seven adults collected by Mr Anderson between 

 5th and loth April 191 3, at Garrynahine, Callernish, western Lewis, the 

 four females are like the Uig specimens, but the males have feeble 

 pectoral spots and but slight trace of an abdominal yellowish wash, 

 while the line of demarcation is rather sharply indicated. The females 

 from Tarbet, Harris, are also like the Uig specimens in colour ; as will 

 be seen from the table at p. 536, both these and the mice from Caller- 

 nish are smaller than the typical series, though the relative size of the 

 feet and ears is the same. 



The skull differs from that of sylvaticus in its larger size, narrower 

 zygomatic, inter-orbital and cranial widths ; the bullae are smaller, the 

 cheek-teeth and diastemata longer — characters which are expressed in 

 the shorter post-molar and longer palatal regions. 



Local variation : — Apart from the reduction of size noted above in 



