574 UNITED STATES GEOLOGICAL SURVEY OP THE TERRITORIES. 



length; inner lip reflexed and appressed in the non-perforate umbilical 

 region; outer lip broadly retreating above, and prominent below; surface 

 nearly smooth, but showing under a good lens very fine, slightly sigmoid 

 lines of growth, crossed by microscopic revolving striae. 



Length of a medium-sized adult, 0.38 inch; breadth, 0.17 inch; length 

 of aperture, 0.14 inch; breadth of same, 0.08 inch; slopes of spire nearly 

 straight, with a divergence of about 30°. 



The flattened, rather numerous volutions, nearly smooth surface, pointed 

 apex, and general physiognomy of tins little shell give it more the appearance 

 of a Eulima than of a Goniobasis. It certainly occurs, however, with 

 decidedly fresh-water types, such as Viviparus and Gojiiobasis, without, so 

 far as known, any admixture of marine forms. I have concluded to refer it 

 doubtfully to Hydrobia ; though if probably does not belong to that genus. 

 Mr. Tryon, on examining a specimen sent to him, thought that it might 

 belong to the genus Asnminia. Possibly I should call it A. eulimoides. It 

 was evidently gregarious, as a single piece of the rock in which it occurs, 

 not more than three inches long by one and three-quarters wide, and less 

 than an inch in thickness, seems to contain not less than one hundred 

 specimens. 



Locality and position. — Clear Fork of Powder River, Montana; from 

 the Fort Union Fresh- and Brackish-water Lignite group (probably Eocene). 



Genus MICROPYRGUS, Meek. 



Synon. — Melania (sp.), Meek and Haydeu (1856); not Lamarck and others. 



Micropyrgus, Meek (1866), in Conrad's Smithsonian Cheek-List N. Aw. Eocene Invertebrate 

 Fossils, 12 and 35. 



Etym. — iunp6c, small ; irvpyoe, a tower. 

 Type. — Melania minutula, Meek and Hayden. 



Shell very small, subcylindrical, imperforate, obtuse at the apex; body- 

 volution small, or less than half the entire length ; aperture rhombic-oval, 

 very narrowly rounded, and more or less effuse; peristome apparently not 

 continuous; outer lip thin, simple, most prominent below the middle. 



This genus is probably nearly related to Hydrobia, but differs from that 

 group, as defined and restricted by Dr. Stitnpson, in its more slender form, 

 obtuse apex, convex volutions, and entirely imperforate axis. 



I am not sure that any other species with which I am acquainted belongs 



