MURIDH—ARVICOLINS—ARVICOLA RIPARIUS. 163 
the same relatively:—The average head and body is to the average tail 
734.55 : 1.50 :: 1: 0.34; that is, the tail is almost exactly one-third as long 
as the head and body. The average fore foot is to the average hind foot 
:: 37: 80; that is, it is a little less than one-half as long. It should be 
observed, likewise, that the variation in the proportional lengths of parts is 
considerable, ¢. ¢., the longest trunk has not always the longest tail and feet, 
nor is a long hind foot always accompanied by an equally lengthened fore 
foot. These calculations, moreover, it should be remembered, really exhibit 
a minimum of purely individual variation; for the nineteen specimens occurred 
under the most similar conditions possible. 
We shall next introduce a table of sixteen specimens, all from the same 
locality, but taken at different seasons, in different stages of growth, and 
prepared by different persons, for which reasons the series may be held to 
fairly express an average of purely individual variability. Yn color, they are 
al so much alike, that no tangible difference can be noted, some being merely 
a shade browner or grayer than others. On an average, they are a trifle 
browner or brighter than the Philedelphia series. 
TABLE XL.— Measurements of sixtcen specimens of A. rpanius collected by J. W. P. Jenks, at Middleboro’, Mass. 
3 Tail to z F 
B Date. Prepared by— ‘St llaeoall = sj Remarks. 
3 7 s . 3 De 
ele SU ee en cee [eine | 
5 K g i! M70. G fs Sal 
A ni | an] BR |e q q wn | 
¢ | Mar. 11,1856} S. G. Brown ..- | 1.40'| 4.285 | 1.80 | 1.98 | 0.37 | 0.75 | The first nine specimens enumerated 
| & | Sept. —, 1855 ]....do ...-...... 1.05 | 4.42 | 1.83 | 2.00 | 0.36 | 0.76 were skinned from alcohol by Mr. 
| 9 | Sept. —, 1x55]....do ........-.. 1.36 | 4.27 | 1.56 | 1.75 | 0.38 | 0.80 Brown, and the first fonr columns 
9 | Sept. —, 1855 eyidop Geass 1.34 | 4.42 | 1.83 | 2.00 | 6.37 | 0.76 of measurements of these nine are 
oe | PSE ts— Goo! mosis Oh <n einen 1.33 | 4.40 | 1.83 | 2.00 | 0.36 | 0.75 his, as taken from the flesh. Some 
| o |\Sept. —, 1855} --..do..2...-2 25. 1.35 4.44 | 1.43 | 1,56 | 0.33 | 0. 74 of his figures for the tails as given 
\ o | Sept. —, 1855 ].-..do .........- | 1.25 | 4.15 | 1.80 | 2.05 | 0.38 | 0.47 on the labels are obviously incor- 
6) || ‘Sept. —, [855')2=-°do)<--= 2-2. 1.34 | 4.42 | 1.83 | 2.00 | 0.38 | 0.75 rect, and probably all are in excess, 
¢o | Spring, 1856}.-. do...-... -. 1.25 | 4.10 | 1.56 | 1.83 | 0.39 | 0. £0 his point being taken too far inside 
Jane 81855] J. W. P. Jenks..|...-. 4.33 | 1.40 | 1.50 | 0.33 | 0.70 the skin. All the other measure- 
MN Ger 20h B00) eer Onsen ay tsi ia me | 4.00 | 1.37 | 1.50 | 0.36 | 0.74 ments are from the dried skins. 
Octye 22 (1855||pseedoinsn essere |eeaee Lyca|| See 0.37 | 0.75 | 
Duly: 9/1855) doses. ce. 4.25 | 1.55 | 1.70-| 0.36 | 0.75 
DAYS Hepa ec) Acesebenen [bees 5.25 | 1.83 | 2.15 | 0.88 | 0 80 | 
Nove 53185522 2-d0).2--25..5-2.|.2 222 5.75 | 1.60 | 1.80 | 0:36 | 0.75 No. 1098 is overstuffed, and Nos, 1432 
NOX Ut Go0|| oa OOlse= aoe kee ae Wee 5.60 | 1.80 | 2.08 | 0.40] 0.80 | and 1099 seem to be somewhat so. 
From this table it appears that the head, from point of the nose to 
the occiput, averages just one and a third inches in length, which is slightly 
less than one-third of the head and body together, and a little less than the 
average length of tail; its variation is 0.15, or a little over ten per cent. of 
