194 PAL^OI^TOLOGY. 



angle of spire nearly or quite regailar, divergence somewhat variable, but 

 usually about 26°. 



This species seems to be very variable, particularly in its ornamenta- 

 tion. On some specimens, both the costse and the revolving lines are quite 

 distinct, while, in others, one or the other, or both, become more obscure, or 

 nearly obsolete. Sometimes the costa3 are rather closely arranged, and in 

 other specimens they are proportionally a little larger and more widely 

 separated. Occasionally they are somewhat more prominent, or even assume 

 a minutely nodular appearance, around the middle of the volutions of the 

 spire and above the middle of the last turn (see fig. 7 (I). Where the costa; 

 fade away, they sometimes, especially on the body-volution, merely pass 

 into coarse, irregular lines of growth. The volutions also vary in their 

 convexity, being sometimes flattened on a line with the slope of the spire, 

 so as to show a merely linear suture (see fig. 7 a), and, in other examples, 

 they are more convex. In one specimen in the collection, the volutions are 

 decidedly convex. This specimen also has the revolving lines and- the 

 costse very nearly obsolete, though small, obscure, closely-arranged costse 

 are seen near the upper part of the spire, and the rather strong lines of 

 growth on the body-volution show a tendency to gather into a few irregular 

 folds, or costse. (See fig. 7 c.) 



There are also in the various Tertiary beds of Wyoming forms more 

 or less nearly like this, ranging through a great thickness of strata, and 

 varying so much as to be with great difficulty separated from each other. 

 In regard to their exact relations to the shell here figured, and to each 

 other, as well as to two species figured by Professor Hall in Fremont's 

 report, and one by Mr. Conrad in the Proceed. Acad. Nat. Sci. Philad., from 

 this region, I have not been able to arrive at any very satisfactor)' con- 

 clusion. Our specimens, however, certainly look very unlike Professor 

 Hall's figures of the species described by him. 



This species resembles more or less nearly several of the existing 

 western forms; but, after careful comparisons, I have been unable to 

 identify it with any of them. Its specific name was given in honor of 

 Col. J. H. Simpson, of the United States Topographical Corps, who brought 

 the typical specimens from the Far West. 



