568 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



Goniobasis? omitta, M. &H. 



Plate 42, flgs. 4, a, b, c. 

 Melania omitta, Meek and Hayden (1857), Proceed. Acad. Nat. Sci. Philad., IX, 136. 



Shell very small, slender, and elongated ; spire attenuate, acute at the 

 apex; volutions about seven and a half to eight, flattened or very slightly 

 convex, increasing gradually in size from the apex; last one prominent, but 

 not distinctly angular around the middle; suture linear, moderately distinct; 

 surface apparently nearly smooth ; aperture rhombic-subovate, rather narrow, 

 somewhat produced and subangular below. 



Length, about 0.34 inch; breadth, 0.11 inch; apical angle regular, 

 divergence about 24°. 



This is a delicate slender little shell, resembling somewhat the last- 

 described species, but it is much smaller, although having the same number 

 of whorls. It is also proportionally more slender, and has its whorls more 

 flattened. It is the smallest and most slender species referred to this group 

 from the Upper Missouri rocks. None of the specimens have the surface of 

 the shell preserved. 



Among foreign fossil species, this shell may be compared with Melania 

 nitida, Lamarck, and M. distorta, Defrance, from the Paris basin. It agrees 

 most nearly with the latter in size, but seems never to be bent like that 

 species, which also has about two more whorls and a more angular aperture. 

 It is smaller than the M. nitida, and has scarcely more than two-thirds as 

 many whorls, while its aperture is not so rounded below. I am far from 

 being satisfied that it belongs to the genus Goniobasis, or even that it is a 

 fresh-water shell, as it is directly associated with Corbula perundata, and 

 apparently a cast "of a small Lucina. It also has much the form of a Eulima, 

 and may yet have to be called Eulima omitta. 



Locality and position. — Same as last ; or possibly in a somewhat lower 

 bed at that place. It occurs in a pebbly sandstone, the pebbles being very 

 small, black, and silicious. 



Goniobasis gracilenta, Meek. 

 Plate i-i, tig. 3. 



Shell small, slender, elongate-conoidal ; volutions about seven, increas- 

 ing gradually in size, convex but not rounded, last one a little produced 



