ir.L' 



MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



Table XXXIX. — Measurements of nineteen specimens of A. RiPAnrcs/mm Philadelphia. 



It appears from (his table that the average length is 4.0.1, and, in 1 he 

 best of our judgment, no allowance for shrinkage in the dimension is to be 

 made. The extremes are from 4.00 to 4.75, a variation of three-fourths of 

 an inch, or about 17 per cent, of the mean. The average of the tail, without 

 its hairs, being 1.42, we may similarly set its true average length at an inch 

 and a half; its variation amounts to 0.37, about the same percentage. The 

 fore foot, averaging 0.35 in its present state, ranges from 0.30 to 0.38, that 

 is, a variation of over 20 per cent.; but from the smallness of the measure- 

 ment, the difficulty of placing the calipers upon exactly the same spot in the 

 several specimens, and an actual difference in the position of the ball of the 

 thumb when this is dried, the true percentage of variation would probably 

 fall below (1.20, and thus be nearly (he same as in the cases of the head and 

 body and tail. The average dimension of the fore foot in life may be set at 

 three-twelfths of an inch, or three lines. The hind foot, from the tuberosity 

 of the heel to the end of the longest claw, can be measured with great exact- 

 ness: it averages <>.77, and so may be fixed at 0.80, or four-fifths of an inch, 

 in life; it varies 0.12, or very nearly 17 per cent., as before. With these 

 calculations for absolute dimensions and variation, we may further consider 



