1G4 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. Tims 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 overstuffing, 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.G6 in length ; but this dimension, 

 as we have remarked, is certainly in appreciable excess. We should not 

 place the true average above 1.00 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 sun 1 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 it 

 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. 10'J8, the largest of the specimens, has feet of 

 only average dimensions. 



As corroborative of the two foregoing tables, and as introducing new 

 data in dimensions, we shall next present a table of sixteen specimens from 

 slightly-separated localities (hut of strictly the same geographical range), 

 and separated from the localities already noticed by a considerable interval. 



