MURIDH—ARVICOLINA—ARVICOLA XANTHOGNATHUS. 203 
the same grounds, though he only admitted it among the hypothetical species 
of his great work. But there is no evidence that the species has occurred in 
the United States, and that it ever does so is highly improbable; and conse- 
quently all the citations of “xanthognathus” from this country—those of 
Godman, Harlan, Say, DeKay, Linsley, and others —are referable only to 
riparius. We have not seen Sabine’s article, where the name appears; but 
Richardson says positively that Sabine’s “xanthognatha” is not this animal at 
all, but is what he (Richardson) calls “pennsylvanica Ord” (see under Arctic 
riparius in this memoir). We are equally in the dark respecting the “Cam- 
’ of Desmarest, which Godman, for instance, refers 
pagnol aux joues fauves’ 
to his ‘‘xanthognatha” (=riparius), but which Audubon and Bachman cite as 
true zanthognathus. Audubon and Bachman have blundered in citing ‘“ xan- 
thognathus Harlan and Godman”; but it seems to have been a mere slip of 
the pen, for they expressly state on a subsequent page that Harlan’s and 
Godman’s animal cannot be the true xanthognathus. 
This Arvicola appears to inhabit most of British and Russian America. 
Audubon and Bachman say they took it in Labrador; Leach got his from 
Hudson’s Bay ; and we have other rather easterly quotations at hand. But 
the creature seems to be especially abundant and characteristic northwest- 
wardly, as in the region of the Mackenzie, Anderson, and Yukon Rivers. 
Notr.—We have a great many skulls of this animal before us, but it 
seems not worth while to tabulate them, as they show nothing whatever dif- 
ferent from those of riparius, excepting a somewhat larger size; all the pro- 
portions are the same. Even the increase in size is only evident on striking 
averages, since the smaller skulls reach well into the dimensions of the larger 
examples of riparius. The skulls run in total length, 1.15 to 1.30; in width 
of zygomatic arches, up to about 0.75; in height, upward of 0.50; at the 
interorbital constriction, 0.15 or 0.20; length of molar series, 0.25 or 0.30; 
length of lower jaw from tip of incisor to back of condyle, nearly an inch; to 
tip of coronoid, about 0.75; the under incisors are 0.30 or 0.40 long from the 
alveoli; the upper have the ordinary relative size. 
The dentition of this species is strictly that of the réparius group, and, 
in fact, so far as we can see, identical with that of A. riparius. There are 
the usual variations in the form of the back upper molar, which, however, 
always shows its crescent and two external lateral triangles; while the front 
under molar has, as in 7éparius, the maximum number of lateral triangles, 
owing to the far advance of the median zigzag line of enamel. 
