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 aK-eolar 

 level. The inferior zygomatic process of the maxillary is a massive 

 plate, the vertical anterior border of w^hich 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 mK 

 The palate is flat ; the maxillo-palatine suture does not reach further 

 forwards than opposite the hinder part of in}-. The ecto-pterygoid 

 plates are well developed ; the pterygoid fossae 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 bullae ; 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 bullae ; 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. 



' 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 least half worn. , 



