INVERTEBRATE PALAEONTOLOGY. 299 



around the outer side below the middle; suture quite distinct in consequence 

 oi' the convexity of the volutions; umbilicus rather small, but deep: aperture 

 very nearly circular; surface ornamented by fine, regular, closely-arranged, 

 revolving stria', crossed by delicate, rather oblique, lines of growth, which 

 are gathered into little, regular, oblique wrinkles near the suture and around 

 the umbilicus; columella not terminating in a point below. 



Length, about 0.26 inch; breadth, 0.28 inch; angle of spire, 68° 

 to 70°. 



This is a neat little shell, still retaining the brilliant pearly luster of its 

 inner layer, while the lines of growth on the exterior are often strong enough 

 to form, with the revolving striae, a delicately-cancellated style of ornamenta- 

 tion. The first specimens studied have the whorls almost regularly rounded; 

 but others, since obtained, show more or less tendency to become subangular 

 around the outer volution below -the middle; while there seem to be all 

 gradations in this character. Our figure 8, a, however, has this angularity 

 a little exaggerated, although the specimen from which it was drawn is 

 unusually angular. Most of the specimens have the volutions as round as 

 represented in figure 9, a. So far as can be determined, there seems to be 

 no angularity at the base of the columella, as often seen in recent species 

 of this genus. In the latter, this character, when it exists, is generally, if not 

 always, caused by the termination of a small revolving ridge around the 

 umbilicus. In the shell under consideration, no such ridge exists; the whorls 

 rounding almost regularly into the umbilicus. 



The large specimen represented by our figure 9, b, is from a different 

 locality, and apparently a somewhat higher position. The figure represents 

 its natural size, which is also that of all the mature specimens from this 

 locality; while figures 8, a, b, and 9, a, represent the mature average size of 

 the shell from other localities. All of the large specimens, such as that 

 represented by 9, b, have the shell exfoliated so as to give only an unsatis- 

 factory idea of the surface-markings, which I am not sure are correctly 

 represented on the figure. Consequently, I have some doubts in regard to 

 the. specific identity of these larger shells with the typical specimens. 



Locality and ■position. — The smaller typical specimens, such as are 

 represented by figures 8, a, b, and 9, «, were found on the Yellowstone River, 

 150 miles from its mouth, in beds containing apparently a blending of the 

 fossils of the Fort Pierre and Fox Hills groups; and the large ones repre- 

 sented by figure 9, b, came from the latter horizon at Long Lake. 



