636 MURIDA—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, pl. 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 (Sriz. Foss. Mamm., 200, 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 (Caé. Brit, Foss., 1854, 360) and Boyd Dawkins 
(Q./.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 
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