138 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



perceptible. The under parts are dull white, much soiled, with a weak shade 

 of the yellowish clay-color of the sides, and the ashy of the bases of the 

 hairs is always more or less apparent. The luteous shade of the under parts 

 is sometimes almost as strong as on the sides, especially across the abdomen. 

 Between the thighs and arms, and under the throat, a whiter and ashier shade 

 prevails. The tail is distinctly bicolor, but not very sharply so; the under 

 surface is like the belly or rather yellower, the upper like the back or rather 

 darker. There are no definite markings about the head ; but a slight dusky 

 area frequently observable about the eyes, and a sort of stripe of dusky along 

 the nose, sometimes suggest a certain particolm-ation there. The upper sur- 

 faces of the hands and feet are nearly white. 



We should not omit to add that the pelage is everywhere long, full, soft, 

 and mollipilose, with but little admixture of lengthened bristly hairs, thus 

 affording efficient protection from the rigors of the winter of high climates. 

 There is a tangible difference in this regard in the more southern varieties. 



We have great pleasure in adding this interesting animal to our fauna, 

 our only previously-recorded form being the var. gapperi, and the Mus rutilus 

 of Pallas being supposed to be confined to the north of Europe and Asia. 

 Of the correctness of our identification there can be absolutely no question 

 whatever. We have carefully compared our North American series with 

 specimens from Lapland and Kamschatka, and they prove identical. All the 

 differences supposed to mark the North American "Hypudseus" disappear in 

 the Arctic series below given, being only applicable to the series from the 

 Northern United States and adjoining regions; and they are, we hold, only 

 indicative of a climatic differentiation. We challenge the proof that Mus 

 rutilus is not a circumpolar species, which, south of a certain isothermal, has 

 become modified into what is known in North America as "Hypudseus gap- 

 peri" and in Europe as "H. glareola" and "H. rubidus". 



Professor Baird says, of the skulls of "Hypudasus" he examined, that 

 that of gapperi "bears a very close resemblance to that of Arvicola rutilus; 

 so close, indeed, that * * I am unable to indicate reliable characters to 

 separate specimens from Massachusetts and Lapland". We are able to 

 include glareola in the same statement, and to prove, by the following table 

 of measurements, that there are no cranial or dental differences whatever in 

 the three supposed species. 



