638 



MURID^— MUS 



and do not afford any noticeable contrast with the general tint of the 

 back. The tail is usually dull brown, both above and below, but some- 

 times the under surface is slightly the paler. 



The young in first pelage hardly differ from the adults in general 

 colour ; their coat is a little softer and closer, consisting chiefly of the 

 woolly underfur. 



The skull (Fig. 93) compared with that of the Field Mouse is dis- 

 proportionately small and peculiarly flattened. In correlation with the 

 small size and weight of the brain (see p. 566 above), the broadly ovate 

 brain-case is depressed, its depth being relatively little greater than that of 

 the rostrum. The interparietal is large and rectangular, and it belongs 

 wholly to the occipital region, the lambdoidal crest and suture passing 

 in front of, instead of behind it, as in A. sylvaticus. The interorbital 

 region is broader than the rostrum ; its edges are square and not 

 beaded, and the temporal crests, which are their backward continua- 

 tions on the sides of the brain-case, are very feebly developed. The 



(I) (2) (3) 



Fig. 93. — Skulls of Mus musculus (l) dorsal, (2) ventral ; AND Mus muralis (3) ventral ; x \\. 



zygomatic arches are relatively strong, and their greatest breadth 

 usually falls just in front of the glenoid articulations; the vertical 

 anterior border of the masseteric plate is situated distinctly in advance 

 of the roof of the infra-orbital canal, and at its base there is a quite 

 small, but prominent peg-like process, from which the tendon of the 

 anterior part of the masseter muscle takes its origin. The incisive 

 foramina are greatly narrowed behind ; they are of remarkable length, 

 and terminate posteriorly about opposite the middle of tn}-. As in 

 Micromys, the palatal shelf extends backwards behind the tooth-rows 

 for a short distance; in this region it shows also a slight median 

 ridge, which is frequently channelled by a weak longitudinal ventral 

 groove. The mesopterygoid fossa is of moderate length, wider behind 

 than in front, where it terminates squarely. The pterygoids are 

 straight, and their hamular processes barely meet the moderately large 

 auditory bullae. The nasals extend forwards but little in advance of 

 the front faces of the incisors. 



The mandible is slightly shorter and deeper relatively than in the 



