MUKIUiE-AKVIOOLlN^-ABVICOLA XANTOOONATUU8. 203 



the Hanie ^roiiiiHo, tlinii^h he only admitted it among the hypothcticuil HpociL-s 

 of his gifiii work, lint there m no eviiUincu that the species has occurred in 

 tlic United States, and that it ever dues ho is highly improimhle ; and conse- 

 quently all the citations of "xanthognathus" from this c(tiintry — those of 

 Godman, Harlan, Say, DoKay, Linsley, and otliers -arc referable only to 

 riparim. We have not seen Sai)ine's article, where the name appears ; but 

 Richardson says (Hisitively that Sal)inc'8 "xanUiognatha" is not this animal at 

 all, but is what he (Riciiardson) calls "pennsi/lvtinica Or<l" (see under Arctic 

 riparius in this memoir). We arc equally in the darU respectiuj} t' •; "Cam- 

 pagnol aux joues fauvcs" of Desmarest, which Godman, for insttmce, refers 

 to his "xanthognatha" {zzriparius), but which Audubon and Uachman cite as 

 true xanthognathus. Audubon and Bachman have blundered in citing ^^xan- 

 th'ignathm Harlan and Godman"; but it seems to have been a mere slip of 

 tlie pen, for they expressly state on a subsequent page that Harlan's and 

 Guduiun's animal cannot be the true xantftognathas. 



'i tt's Arvicola appears to inhabit most of British and Russian America. 

 Audubon and Bachman say they took it in Labrador; Leach got his from 

 HudsonV Bay ; and we have other rather easterly quotations at hand. But 

 the creature seems to be especially abundant and charocteristic 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 

 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 riparius group, and, 

 in fact, so far as' we can see, identical with that of A. riparius. There are 

 the usual variations in the tbrm of the back upper molar, which, however, 

 always shows its crescent and two external lateral triangles ; while the ti'ont 

 under molar has, as in riparitu, the maximum number of lateral triangles, 

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



