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 71, 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. Beene 
Dimensions in millimetres (from actual speci- a ae Mio 
mens) :—Old adult, 125—-36—19—12,; adolescent, 115 agrestis exsul. 
—32—15—12; immature, 100—30—18—II ; youngest 
examined, 60—15:5—1I4—5-5. 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, 
14-6 to 16-5 ; least inter-orbital, 3-2 to 3-5 ; mastoid, 11-6 to 12-5 ; occipital 
depth (median), 6-2 to 7-1; length of nasals, 6-8 to 8-4; of diastema, 
7-4 to 8-1; of maxillary tooth-row, 6-5 to 7-0; of mandible, 16-5 to 18-0; 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 

1 Small but distinct in nine; recognisable microscopically in eleven ; entirely 
absent in one. This loop is also found occasionally in the North American /. 
pennsylvanicus (Ord), as figured by Miller, WV. Amer. Fauna, No. 12, 23rd July 1896, 
fig. i. I have also figured it as an abnormality in M. a. agrestis (Proc. Zool. Soc., 
London, 1896, 598). 
