147 
% 
cranial peculiarities of No. $238, Mus. Smiths., from California). 
—Newberry, P. R. R. Rep., vi, 1857, Zool., 61. 
1857. Arvicola medesta, Baird, M.N. A., 1857, 5385 (based on No. y3'y7, from 
Rocky Mountains; very young). | 
1857. Arvicola rufidorsum, Baird, M. N. A., 1857, 526 (Holmes’ Hole, Mass ; 
a very red specimen). 
1857. Arvicola brewert, Baird, M. N. A., 1857, 525 (bleached breed from 
Muskeget Island, Mass). 
1862. Hypudeus riparius, Maximilian, Arch. Naturg., xviii, 1862; Verz. 
N. A., Saug., 1862, 174. 
1874. Arvicola (Myonomes) riparius, Coues, Proc. Acad. Nat. Sci., Phila., 
1874, 189; Mon. N. A. Rodentia, 1877, 156.—Coues and Yarrow, 
Zool. Expl. W. of 100th Merid., 1876, 106. | 
Description of the Species.—Average length, 435; tail, 1.5; fore foot, 3 
lines; hind foot, from tuberosity of heel to end of longest claw, four fifths 
inch. 
Gray-brown, darker along the middle of the back, especially toward 
the tail. Beneath, the plumbeous hairs are tipped with white, resulting 
in a hoary-ash, whitest on the breast and belly, darkest on the throat. 
There is no line of demarcation between the upper and under parts. 
The typical coloration given above is departed from in three directions : 
(1) toward red, the extreme of which is reached in the type of ‘“‘rufidorsum,” 
(2) toward black, including certain northern Illinois specimens, having 
long, dense, and almost fluffy pelage, forming the variety “ longijulis,” 
but the fur of this species varies so much in these points with age, health, 
season of the year, and climate, that “longipiles” can not be regarded as 
even a permanent variety; (8) lastly, the color tends to gray, the ex- 
treme of which has passed as “brewert.” This form has only been found 
on a low, sandy island, between Nantucket and Martha’s Vineyard, and 
is believed by Mr. Allen to be only a thoroughly sun-bleached form of 
riparius, the scattered grass of the island giving but little shelter from 
the sunlight from above, and the hot, light colored sands finishing the 
bleaching of the under parts. The mice living in the sand dunes of 
Ipswich show the same half-white appearance of the Massachusetts speci- 
mens. Indeed, the present species is, according to Dr. Coues, subject to 
almost endless variation of size, color, 2nd proportion of parts. The same 
authority closes an elaborate survey of this species, extending over thirty 
quarto pages, and including results of examinations of over one hundred 
and fifty specimens: ‘We have no alternative, then, but to throw all 
the names together as indistinguishable synonyms of riparizs. In 1857 
many names, all doubtless supposed by their several proposers to indicate 
valid species, were either formally or virtually suppressed ; and a further 
