204 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
?ARVICOLA RICHARDSONI of Aud. & Bach. 
Large Northern Meadow Mouse. 
“ Arvicola riparius ORD”, apud RICHARDSON, Fn. Bor.-Am. 1829, i, 120. (Not of Ord.) 
dArvicola richardsoni, DEKay, N. Y. Zool. i, 1842, 91.—Aup. & Bacu., Q. N. A. 1853, iii, 163, pl. exxxv, f. i 
(based on Richardson’s animal).—Barrp, M. N. A. 1857, 551 (same as the foregoing). 
Selecting a number of the very largest skins in the collection, we can 
see that they noticeably surpass the average of United States riparius, and 
stand at, if not beyond, the maximum of the latter. But this is true of only 
a small proportion of our lot; the others shade insensibly down to the aver- 
age of riparius ; and all these largest ones are accompanied by others of much 
less stature, taken living side by side, and certainly not specifically different. 
The difference in the whole series, moreover, is not greater than we demon- 
strate in the case of unquestionable A. zanthognathus. 
All the remarks we have offered under head of A. xanthognathus have 
been based upon unquestionable examples of that form. But, after elimin- 
ating these, we find a considerable number of specimens that are precisely 
like ordinary zanthognathus, yet have no trace of the chestnut cheeks. ‘They 
are clearly not referable to ordinary riparius, and are equally far removed from 
the small var. borealis. 
We puzzled long over these before we were led, we believe correctly, to 
refer them to xanthognathus. Among Mr. Kennicott’s extraordinarily full 
series of xanthognathus we found here and there a skin showing no chestnut 
cheeks, yet which he had labeled with his autograph ‘‘xanthognathus”. This 
excited our suspicion; for we had noted with pleasure that, of the many hun- 
dred mice of all sorts contributed to the collection by this eminent naturalist, 
not one had been labeled by him of which there was the slightest doubt, and 
his labeling has proved in every case correct. There is little if any doubt 
that zanthognathus, under certain conditions, does not acquire the chestnut 
cheeks—in short, that a certain proportion of specimens shade into riparius. 
This goes far to show that the two forms are not specifically distinct. We 
tabulate below these doubtful skins. It will be seen by the measurements 
that they represent an animal at and beyond the maximum of ordinary riparius 
in size, with comparatively shorter members; and the skins show a certain 
undefinable coloration and condition of pelage which assures us they belong 
nearer canthognathus than riparius ; but their positive determination is to us 
at present impossible. 
a 
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