THE FIELD MOUSE 513 
suture. The inter-orbital region is moderately constricted, about equal 
in width to the hinder part of the rostrum. The zygomatic arches are 
very slender, with delicate splint-like jugals; they are but slightly 
expanded, and drop rapidly from their maxillary roots to the alveolar 
level. The inferior zygomatic process of the maxillary is a massive 
plate, the vertical anterior border of which projects considerably in 
advance of the slender bridge-like upper process. The nasals are long 
and slender; they project noticeably in advance of the incisors, and are 
supported anteriorly on blade-like processes of the premaxillaries. 
The diastema is nearly twice as long as the tooth-row; the incisive 
foramina are long and reach back to a point opposite the front of 2. 
The palate is flat; the maxillo-palatine suture does not reach further 
forwards than opposite the hinder part of m!. The ecto-pterygoid 
plates are well developed; the pterygoid fossz are shallow and slope 
backwards from the level of the palate to that of the basi-sphenoid ; 
the inter-pterygoid space is long, narrow, and rectangular. The hamular 
processes curve outwards behind, and are in contact with the eustachian 
expansions of the small rounded bull ; each bulla has a large meatus, 
the rim of which is slightly tubular. The basi-occipital is of moderate 
width, with a low median ridge and shallow lateral depressions; the 
paroccipital processes are very small; the body of the presphenoid is 
reduced in part to a thread of bone. 
Adult skulls from different parts of the British Isles show, when 
carefully examined, slight but quite tangible differences of form and 
proportion, which indicate the tendency of the Field Mouse to develop 
races adjusted to the requirements of purely local conditions. Compared 
with skulls from central and southern England, those from Wales are 
slightly deeper and narrower, with smaller bullae and slightly shorter 
post-molar and diastemal lengths ; those from the lowlands of Scotland 
are relatively narrower throughout, with more depressed brain-cases and 
rather small bullz; those from the Highlands are small, with slightly 
larger brain-cases, wider nasals, longer diastemata, and incisive foramina, 
Irish skulls agree more closely with those from England and Wales 
than with those from Scotland. Those from Man have slightly deeper 
brain-cases and longer palates. On Clare Island and Inishmore the 
skull seems to be considerably larger than usual, and this is the case 
also on Alderney and Scilly. Skulls from Jersey agree with the 
largest English specimens in size, but in several of their relative 
proportions they resemble those of the Scotch Highlands. For further 
details and summary of relative dimensions, see Hinton, Ann. and Mag. 
Nat. Hist., July 1914, 118, and the table at p. 518! below. 
1 The measurements in this table (and those in that at p. 538) were made with 
great care and good instruments. The skulls measured were in all cases adult, the 
cheek-teeth being at /east half worn. 
