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142 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



EVOTOMYS RUTILUS GAPPERI, (Vig.) Coues. 



Bed-backed Honsc. 



ArvicoTa gapperi, Vigors, Zool. Journ. v, 1830, 804, pi, 9 (Canada).— DeKay, N. Y. Zool., i, 1843, W.— 

 ScniNZ, Synop. Mam. ii, lt<4S, 3.'i2.— Allen, RiiII. Has. Coinp. Zool. i, 1H69, 831. 



Arvicola {Hi/pudceus) gapperi, Bairo, M. N. A. 1857, 518. 



EfOtomys rutilM gapptri, CoUKS, Proc. Aoad. Nat. Sci. Pliila. 1874, 187. 



Arvicola fitha, Aud. &. Bach., Jonni. Acad. Nat. Sci. Phila. viii, 1842, pt. ii, 895 (name pre-occnpied and 

 afterward cbanged to " dcAoyi ").— VVaonxr, Wiegniann'H Arcbiv, 1H43, pt. ii, 53. 



Arvicola dekayi, AuD. & Bach., Q. N. A. iii, 1854, 887 (same as their fulra of 184'.!), (exclude tbe synonym 

 "oneida De Kay", whicli belongs to Arvicola riparim). 



Diagnosis. — A. rutilo similUmus, sed staturd paululum major, colorihus 

 ohscuriorihus, caudd, pedibus auriculisque longioribus. 



Habitat. — The northern frontier of the United States, fro- Atlantic to 

 Pacific, and an adjoining belt of British America; further north replaced by 

 the true rutilus. Nova Scotia. South to Massachusetts. 



The occurrence of the true A. rutilus in North America was not more 

 unexpected to us than the relationship of A. gapperi, which we had always 

 unquestioningly taken as a valid species, proved to be. The differences, as 

 detailed by Baird (op. cit. 521), seemed perfectly satisfactory; but, with more 

 exten.sivc material than that writer enjoyed, we are enabled to make out a 

 different stiite of the case. Our views would have received strong corrobo- 

 ration upon a priori considerations from the mere circumstance of finding 

 the true rutilus in America; but, independently of this, we are prepared to 

 present a chain of evidence that cannot be broken down, from direct com- 

 jiarison of specimens irrespective of locality. 



The general tendency of animals that range from temperate to frigid 

 regions, to shorten their members, or, as it were, withdraw peripheral parts 

 from the cold, and to put on thicker, warmer covering, in higher latitudes, is 

 a well-known law, of which the present case merely affords another example. 



We have already seen the condition of the pelage, and the proportions 

 of the tail, ears, and feet, in true rutilus, from high latitudes; and the present 

 variety, witli the same general characters as rutilus, differs in the length of 

 its several members, and their amount of hairiness, as well as in the condi- 

 tion of the general pelage. The difference, however, is not abrupt, nor is it, 

 ill fact, even well marked, except in its extremes. The transition from typi- 

 cal rutilus to tiie extreme of southern gapperi '.a so gradual and insensible 

 that there is no break in the series. This will be evident from the table 

 given lielow, in whicli the various examples of gapperi differ as much among 

 Ihenisulves as some of them do I'rom true rutilus. Still, comparison of the 



