MR. M. A. C. HINTON ON HEBRTDEAN MAMMALS. 833 



tooth has a distinct and often rather large fourth inner angle. As. 

 is usual in niicrotine subspecies, the subspecific cranial characters, 

 first become obvious in old age, although, if care be taken to 

 compare only skulls of equal age, close observation will find the 

 beginnings of such characters in younger stages too. The oldest 

 skull, the type (ISTo. 102), is fairly comparable with that of No. 139,. 

 a M. a. exsul from Mull, from which it is distinguished by its deeper 

 rostrum, lighter jugals, and more nearly vertical occiput ; because 

 the occiput is less sloping the interparietal is a little less 

 reduced, — its posterior border is straight instead of sinuous, 

 and it is rather longer antero-posteriorly. The most striking 

 difference is seen, however, in the form of the " shield " or 

 flattened dorsal area delimited by the temporal ridges upon the 

 sides of the braincase. In exsul (text-fig. 138 a) that part of the 

 temporal muscle which arises between the interorbital region 

 and the glenoid articulation continues apparently throughout 

 life to extend its area of origin by creeping up the sides of the 

 braincase and so pushing the temporal ridge backward ; the result 

 is that in old age the anterior width of the " shield," taken 

 between its antero-external angles, is considerably less than its 

 glenoid width : in the Islay mouse the upward creeping of the 

 temporal muscle ceases at a relatively early stage of growth, and 

 the result is that in old age the " shield " presents a form which 

 is found only in relatively eai'ly stages of development in exsul. 

 The oldest skull of exsul from South Uist (Edinburgh, No. 300) 

 is younger than the skulls from Islay and Mull just described, 

 the temporal ridges not yet being quite fused in the interorbital 

 region, but the form of the "shield" is already more modified 

 than in the aged type of the present subspecies. 



MiCROTUS AGRESTIS NEGLECTUS JenynS. 



Three, Bute. . Dimensions : — 



Head & Hind 



body. Tail. foot. Ear. 



No. 67, male, juv., 19 April 1912 98 26 17 12 



3, female, „ 12 March „ 100 26 16 11 



64, „ „ 18 April „ 95 24 16 11-5 



For cranial measurements see Table at p, 834, 



The short tails are, no doubt, due to immaturity. 



Making allowance for the yovith of these specimens, wa 

 can detect no character by which the skins or skulls can be 

 differentiated from those of M. agrestis neglectus of the mainlands 



In his recently published ' Catalogue of the Mammals of 

 Western Europe ' (1912), Mr. Miller treats the various agrestis- 

 like forms as subspecies. We think it preferable, somewhat 

 as proposed by Barrett-Hamilton in 1896 (P.Z.S. 1896, 19th May,. 

 pp. 599-608), to regard these forms as belonging to two species, 



