636 MURID^— MUS 



This species owes its present almost universal distribution to its 

 success as an invader and colonist of human dwellings and store-places, 

 and to subsequent accidental transport with human commerce. In cool 

 climates, although often found living out of doors, it is rarely met with 

 far from houses or other scenes of human activity. But in countries 

 where the climate is suitable and food readily obtainable, as in many 

 of the warmer parts of America, it has resumed a free or natural 

 station, and competes successfully with the indigenous rodents. In 

 such situations it, like other murines, shows an inherent plasticity, 

 enabling it to develop races modified in one way or another to meet 

 the peculiar requirements of a foreign environment. 



Distribution in time : — The remains of " mice " recorded by Buck- 

 land [Rel. Diluv., 19, 265, pi. xi., figs. 7-9) from the Kirkdale Cave were 

 probably remains of Apodemus ; the figured jaw agrees in size with 

 that of the Field Mouse, and the rather inaccurate drawing of the cheek- 

 teeth might represent teeth of that species quite as well as those of the 

 House Mouse. Owen {Brit. Foss. Mamm., 209, fig. 79) also figures a 

 jaw from Kirkdale ; this drawing, as regards the teeth and form of the 

 jaw, agrees better with the House Mouse, although the size is rather 

 large. The species is listed from Kent's Cavern and the Durdham 

 Down Cave, by Morris {Cat. Brit. Foss., 1854, 360) and Boyd Dawkins 

 {Q.J.G.S., XXV., 198, 1869); it has also been doubtfully recorded from 

 the Pleistocene of Copford by R. Bell {Proc. Geol. Assoc, ii., 217, 1871). 

 It has been stated to occur in the Pleistocene deposits of the Thames valley 

 (see Lydekker, 189). Although we have had the advantage of studying 

 a far greater number of British fossil mouse remains than has any other 

 observer, we have never met with the slightest trace of this species 

 among them ; we are, therefore, inclined to doubt the identifications in 

 some cases, and to think in others that the remains were compara- 

 tively recent introductions in the deposits whence they have been 

 recorded. 



Description : — The House Mouse is a slenderly built, rather sharp- 

 faced murine of medium size (head and body, 75 to 100 mm. ; hind 

 foot, 17 to 19-4; condylo-basal length of skull, 19-8 to 22-4 mm.), with 

 the tail about as long or longer than the head and body, clad with soft 

 fur, and usually of a brownish-grey colour. 



The eyes are small, and somewhat protruding, although much less 

 prominent than in the Field Mouse. The broadly ovate ears are of 

 moderate size, their length being about half that of the head, and they 

 cover the eyes when laid forwards ; save for the naked internal basal 

 portions, they are thinly clothed within and without, with short and 

 fine hairs ; in each the meatal valve is represented merely by a low 

 ridge placed just behind the meatus. 



In each hand the thumb is a vestigial tubercle, scarcely exceeding 



