566 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



ally larger and more ventricose body-whorl, less acutely conical spire, and 

 less flattened whorls, more impressed suture, &c. I am not quite sure that 

 it is a true Goniobasis, as it has its aperture rather less angular at its base 

 than is common in that genus. Our figure 12, b, however, represents a 

 specimen with its aperture rather unusually large, and more than naturally 

 rounded below. The foregoing wood-cut gives more nearly the genera 

 appearance of the aperture and other characters of this shell.* 



Locality and position. — Fort Clarke, and at mouth of Yellowstone River ; 

 from the Fort Union Lignite group. 



Goniobasis teniiicariiiata, M & H 



Plate 43, figs. 14, a, b, c. 



Melania teriuicarinata, Meek and Hayden (1857), Proceed. Acad. Nat. Sci. Philad., IX, 137. 

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

 Fossils of N. Am., 12. 



Shell narrow-subovate ; spire conical, of medium height, pointed and 

 not eroded at the apex ; volutions six, very convex, obliquely flattened above 

 and angular around the middle; suture well defined ; surface ornamented by 

 fine obscure lines of growth, which are crossed on the middle of the whorls 

 by from, three to four or five prominent revolving lines, or narrow carinas, 

 and on all parts of the shell excepting near the apex of the spire, by traces 

 of very fine, indistinct, revolving strias ; aperture ovate, angular above, nar- 

 rowly rounded, and very faintly sinuous below; lip sharp, slightly prominent 

 below the middle; columella gently arcuate. 



Length, 0.53 inch; breadth, 0.30 inch; length of aperture, 0.23 inch; 

 breadth of same, 0.18 inch; apical angle variable with age, mean divergence 

 about 45°. 



The upper of the revolving keels is a little larger than the others, and 

 generally so prominent as to give the whorls a shouldered appearance. Near 

 the apex of the spire, these carinas become obsolete, and on the lower whorl 

 all those below the upper two diminish in size from above downward, so 

 that the fourth and fifth ones are scarcely larger than the other revolving 

 lines seen on the lower part of this volution. Like the foregoing species, 

 this shell varied much in form at different ages, young specimens having the 



* In some respects this species resembles Pachi/chilus, Lea ; and possibly I would be nearer right 

 to call it Pachychilus Nebrascensis. None of the specimens, however, seem to have the outer lip thick- 

 ened :is in that genus, though in some of the larger ones the inner lip is thick. 



