THE HEBRIDEAN GRASS MOUSE 435 



The fur is soft and thick, the longer hairs of the back reaching a 

 length of about 15 mm. in winter and the ordinary hairs about 12 mm., 

 of which their light-coloured tips amount to about 2-3 mm. 



The colour of the upper side is a clear brown, somewhere between 

 "bister" and "ochraceous buff," yellower on the sides. The longer 

 hairs are shiny black ; the under-fur has tips 2 mm. long of dull — 

 between ochraceous and cream — buff, the extreme tips often dusky. 

 The under-side and feet are rather heavily washed with brownish, often 

 light ochraceous, buff. The tail is bicoloured, especially in winter, when 

 all the colours are lighter. 



The skull is large and in old individuals strongly angular, develop- 

 ing a knife-like ridge in the inter-orbital region. The brain-case is 

 short, the distance from the back of the inter-orbital constriction to the 

 posterior surfaces of the condyles being barely equal to the zygomatic 

 breadth. In old skulls the " shield " between the temporal ridges is 

 considerably narrower anteriorly between its antero-external angles 

 than at the level of the glenoid articulation (Fig. 66, 3). 

 In the teeth a small posterior fourth inner angle is 

 usually present in m^, having been found by Miller in 

 ten out of fourteen skulls examined, by Hinton and 

 myself in twenty out of twenty-one skulls ; ^ in other 

 sub-species this formation is very rare, being present 



in only eight amongst one hundred and thirty-six skulls. 



r.. • ■ •„■ ^ /c i ^ ■ F'G. 67.— Right 



Dimensions in millimetres (from actual speci- ^1 ^^ Mkrotus 



mens): — Old adult, 125 — 36 — 19 — 12; adolescent, 115 agrestis exsul. 



—32 — iS — 12; immature, 100 — 30 — 18 — 11; youngest 



examined, 60 — 15-5 — 14 — 55. The tail ranges in subadult to old 



specimens from 28 to 49. Females are found pregnant or nursing 



when they attain 109, before assuming adult pelage. 



Skull: — Condylo-basal length, 26-5 to 28-6; breadth, zygomatic, 

 146 to 16-5 ; least inter-orbital, 3-2 to 3-5 ; mastoid, 1 1-6 to 12-5 ; occipital 

 depth (median), 6-2 to 7-1 ; length of nasals, 6-8 to 8-4; of diastema, 

 74 to 8-1 ; ofmaxillary tooth-row, 65 to 7-0; of mandible, 165 to i8-o; of 

 mandibular tooth-row, 6-4 to 7-0. (Except the least inter-orbital breadth, 

 which decreases with age, the minimum values are those of immature, 

 the maximum those of adult skulls, the subadults ranging between these 

 extremes.) 



Status : — This mouse is a moderately differentiated sub-species of 

 the Skandinavian Grass Mouse, from which, as stated above, it can 



' Small but distinct in nine ; recognisable microscopically in eleven ; entirely 

 absent in one. This loop is also found occasionally in the North American M. 

 pennsylvanicus (Ord), as figured by Miller, N. Amer. Fauna, No. 12, 23rd July 1896, 

 "g- '• I have also figured it as an abnormality in M. a. agrestis {Proc. Zool. Soc, 

 London, 1896, 598). 



