164 MONOGRAPHS OF NORTH AMERICAN RODENTIA. . 
the mean. The average length of the nine specimens, as measured fresh, is 
4.30, and of the seven dried ones is 4.75; the result 4.52, as the true length 
of the animal, is slightly in excess of that deduced from the Philadelphia 
series, and precisely what we should expect from the increase in latitude. 
But before considering the limits of variability in total length of this series, 
we must allow for the probable changes produced in stuffing. Thus No. 
1578, which was 4.10 long when fresh, measures in its present state only 
3.75, without obvious ‘bunching up” of the skin; while No. 1098, with 
apparently moderate overstufling, reaches 5.75, and it doubtless was at least 
5.00 in life. Making due allowance for these contingencies, it will be quite 
safe to assume one inch in length of head and body as the fair average varia- 
bility of mature individuals living in the same locality. It is beyond doubt 
that this capacity for variation will be found exemplified in every considerable 
series of adult specimens from any locality. 
The tails of the above series average 1.66 in length; but this dimension, 
as we have remarked, is certainly in appreciable excess. We should not 
place the true average above 1.60 at most, and this would correspond pre- 
cisely with the slight increase in average size of the Massachusetts over the 
Philadelphia series; the tail being, as before, one-third as long as the head 
and body. We have, however, a higher rate of variation in the length of the 
tail, viz, from 1.37 to 1.80 at least, which is almost half an inch. Although 
this may seem excessive, and be thought not quite reliable, owing to what we 
said of the measurements as taken, yet we are sure it is nothing unusual, 
and, in fact, we demonstrate further on, from consideration of more material, 
a greater variability of the tail than this. The pencil of hairs at the tip is a 
very inconstant feature, depending upon the general condition of the animal 
as to pelage, and perhaps upon season. It averages about a fourth of an 
inch, and ordinarily ranges between 0.10 and 0.30. The measurements of 
the feet, in the above table, are essentially the same as those of the Phila- 
delphia series, and need not detain us, though we may gather the fact that 
increase in total stature is not always accompanied by corresponding increase 
in these members, since No. 1098, the largest of the specimens, has feet of 
only average dimensions. 
As corroborative of the two foregoing tables, aud as introducing new 
data in dimensions, we shall next present a table of sixteen specimens from 
slightly-separated localities (but of strictly the same geographical range), 
and separated from the localities already noticed by a considerable interval. 
