56 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
the average of the rest, and about equals, or rather falls a little short of, the 
minimum of Zeucopus. In length, no full-grown skull of dewcopus 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 /eucopus 
(0.60). It is probable that cadifornicus would just overreach, on an average, 
the maximum of dewcopus; 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 dewcopus 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 series of eighty-two 
specimens, collected at all seasons m 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. 
TaBLeE X.—Measurements of eighty specimens of HesrprroMYS LEUCOrPUS, from Middleborough, Mass., col- 
lected by J. W. P. Jenks. 
INOsoItO Tae 
: Date, eV 2e ane ooo 
| 3 ee Ey E: Occi- Tail Ver- HeGne % zz 
5 iA ye. ate put. ar ebro 02s) = 5 
A |n ico x ica) 
1855. 
2713 |....| Aug. or Sept---| 0.55 | 0.95 |. 1.16 | 3.55 | 2.83 |...-.. 0.36 | 0.80 }....-- Alcoholic. 
2714 Gh) sAaoscecé OF5O 805925 ea | Sale ere Jagooss OSS a ONSanle-eeee Alcoholic. 
O27 yp [ee el |e ¢ (ese eae 0.50 | 0.90 | 1.14 | 3.13 | 3.25 |...--- ON36 |) (02;80! | 2a ee= Alcoholic. 
2716 |.. Conse 0.50 | 0.90 DALAT P3230) [9SE00" | ace 0538) |) 080) |-222=- Alcoholic. | 
| 2717 Mots.-2 5285. 0.50 OL Wedd. arSa FSP 15) poe 0.35))| OS SLi |pe2e2: Alcoholic. 
2720 dob 5-3 0.50 |-0.90 | 1.14 |-3.30 | 3,10 |...... 0.34 | 0.80 |..-.-. | Aleoholie, 
2721 Os ae OOS LONG SELON) lide coe |eebee Ol aera N40 OE Bicone | Alcoholic. 
2722 (Hohe Saas OZ SOS 3) PAL MS: tests 0 oeek On ae OSGi) O80. ses. | Alcoholic. 
