192 Trans. Acad, Set. of St. Louis. 



to the suture, flat on the inner volutions, and slightly sloping 

 inward on the outer volutions ; aperture straight above, cir- 

 cular below. 



«* Surface marked by fine, closely arranged striae of 

 growth.'' 



Remarks. This species is rather a common one in the 

 oolite bed at Burlington, and exhibits some variations in its 

 characters which are not mentioned in the original definition. 

 The chief of these is in the elevation of the spire, it being 

 slightly elevated in some specimens while in others it is 

 slightly depressed below the plane of the outer volution. 

 The angular character of the upper side of the volutions is 

 oftentimes more or less obscure, the cross section of the 

 volutions being almost circular. The umbilicus is broad and 

 in it all the volutions of the shell are exhibited. The 

 specimen here illustrated is perhaps a little smaller than the 

 average adult size, the shell sometimes attaining a diameter 

 of 45 mm. 



Strophostylus bivolve. (W. & W.). 



PL XVII. f. 4-5. 



Original description. ** Shell small, ventricose, composed 

 of about two closely coiled, rounded volutions, spire not ele- 

 vated above the surface of the outer volutions. Inner whor 

 minute, outer volution more rapidly expanding and ventricose. 

 Section of the volution transversely ovate, narrowest at the 

 inner or ventral margin ; border of the aperture with a shallow 

 sinus on the upper side, and another below the middle. Sur- 

 face marked by fine transverse striae, parallel to the border 

 of the aperture." 



Greatest diameter of the type specimen, 20 mm., greatest 

 width of the outer volution of the same specimen 14 mm., 

 height of aperture 11 mm. 



Remarks. The specimens of this species labeled ** types" 

 in the University of Michigan collection, are five in number, 

 two of them being from the Chonopectus sandstone and three 

 from the oolitic limestone. One of the Chonopectus sand- 

 stone specimens is not even cogeneric with the other indi- 

 viduals and may be a specimen of N'aticopsis depressus Win., 



