MUKIDiE— AKVICOLlNiE— ARVICJOLA XANTHOGNATIIUS. 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 "pmnsylvanica Ord" (see under Arctic 

 riparius in this memoir). We are equally in the dark respecting the "Cam- 

 pagnol aux joues fauves" of Desmarest, which Godman, for instance, refers 

 to his "xanthognatha" (=riparius), but which Audubon and Bachman cite as 

 true xanthognathus. 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 

 God man'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. 



Note. — 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 

 iuterorbital 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 riparius 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 riparius, the maximum number of lateral triangles, 

 owing to the far advance of the median zigzag line of enamel. 



