INVERTEBRATE PALAEONTOLOGY. 567 



spire much less elevated and the body-whorl proportionally larger than adult 



individuals. 



Some varieties of the last-described species, on which a few of the 

 revolving lines near the middle of the whorls are larger than the others, bear 

 so much resemblance to this shell as to leave some doubts whether this may 

 not also prove to be only an extreme variety of that species. I have, bow- 

 ever, been able fo separate all the specimens yet seen of these forms without 

 much difficulty. 



Locality and position. — Near Fort Union, Montana, on the Missouri ; 

 from the Fort Union Lignite group. 



Goniobasis siiblaevis, M. &H. 



Plate 42, figs. 5, a, b. 



Melama sublcrvis, Meek and Haydeu (1857), Proceed. Acad. Nat. Sci. Pliilad., IX, 136. 

 Goniobasis sublwris, Meek (1866), in Conrad's Smithsonian Check-List N. Am. Eocene Invertebrate 

 Fossils, 12. 



Shell elongate-conical; volutions about seven or eight, slightly convex; 

 suture rather shallow ; surface apparently smooth, but, when examined under 

 a lens, seen to be marked by very fine, nearly obsolete lines of growth, which 

 are sometimes crossed on the lower whorls by obscure traces of fine revolv- 

 ing strise ; aperture e^al, very narrowly rounded below ; columella faintly 

 sinuous. 



Length, about 1.04 inches; breadth, 0.38 inch; apical angle regular, or 

 slightly concave, divergence from 22° to 24°. 



The polished surface and more attenuate spire of this species, together 

 with the entire absence of revolving lines visible without the aid of a lens on 

 any part of its surface, are characters that will distinguish it from all the 

 other forms resembling it in other respects yet known from these rocks. It 

 differs from all the otherwise similar recent species with which I am 

 acquainted, in not having the upper whorls costate or carinate, and in its almost 

 polished surface. The specimen represented by figure 5, a, has the outer 

 lip broken so as to give an unnatural angularity to the base of the aperture. 

 That represented by figure 5, b. shows more nearly the natural form of the 

 aperture. It may not belong properly to the genus Goniobasis. 



Locality and position. — Mouth of Judith River, Montana; from the 

 Fresh- and Brackish-water Lignite beds of that locality. 



