147 



cranial peculiarities of No. 5II4, Mus. Smiths., from California). 



—Newberry, P. R. R. Rep., vi, 1857, Zool., 61. 

 1857. Armcola modesta, Baird, M. N. A., 1857, 536 (based on No. jVtt i ff o"^ 



Rocky Mountains ; very young). 

 1857. Arvicola rufdorsum, Baird, M. N. A., 1857, 626 (Holmes' Hole, Mass ; 



a very red specimen). 

 1857. Arvicola brtweri, Baird, M. N. A-, 1867, 625 (bleached breed from 



Muskeget Island, Maes). 

 1862. Hypudceus riparius, Maximilian, Arch. Naturg., xviii, 1862; Verz. 



N. A., Saug., 1862, 174. 

 1874. Arvicola (Myonomes) riparim, 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, 4 36; tail, li5^ 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, th-e 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 coloraition 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 '^ longipilis," 

 but the fur of this species varies so much in these points with age, health, 

 season of the year, and climate, that "longipilis'" can not be regarded as 

 even a permanent variety; (3) lastly, the color tends to gray, the ex- 

 treme of which has passed as "breweri." 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, and 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 riparius. In 1867 

 many names, all doubtless supposed by their several proposers to indicate 

 valid specie?, were either formally or virtually suppressed ; and a further 



