56 



MONOdliAPLIS OF SOUTH AMERICAN KODENTIA. 



the average of the rest, and about equals, or rather falls a little short of, the 

 minimum of leucopus. In length, no full-grown skull of leucopus drops to 0.90, 

 while the average of michiganensis is only 0.94 ; and the average length of the 

 under jaw of michiganensis is similarly just under the minimum of leucopus 

 (0.00). It is probable that californicus would just overreach, on an average, 

 (lie maximum of leucopus; but, with only one specimen before us, we cannot 

 declare this to be so. All the other skulls in the lot fall entirely within the 

 usual limits of variation of leucopus proper. Here the range is from 0.94 to 

 1.10, as minimum and maximum, with an average of barely over 1.00. 

 The zygomatic width of the skulls is just about half their length; it ranges 

 from 0.45 to 0.57. The height of the skulls, measured as already explained, 

 is about 0.37 ; the length of the lower jaw is about 0.62; the interorbital 

 width runs a trifle under 0.20. 



We open the discussion with the examination of a scries of eighty-two 

 specimens, collected at all seasons in the same locality. This will certainly 

 give us the individual variability of the species, — its normal flexibility, inde- 

 pendent of those topographic or climatic influences which tend, when opera- 

 tive, to bend it into geographical differentiation. The measurements are 

 reliable, at least half of them being taken from alcoholic specimens; while 

 the dried skins were prepared with especial care by Mr. Jenks. Neverthe- 

 less, we shall make liberal allowance (see below) for taxidermal defects and 

 other sources of error in calculating our limits of natural variation, as well as 

 in deducing the normal standard of size and proportion of parts. 



Table X. — Measurements of eighty specimens of Hesperomys LEUCOPUS, from Middleborovgh, Mass., eol- 



Jected by </. W. P. Jenks. 



