312 Monograph of the Gasteropoda. 



(liminisliing from the aperture ; siibcarinatecl upon the back ; whorls 

 contiguous, but not involute. 



Microceras inornatus — (Hall, 1845). 



Sliell very small ; volutions about two, rapidly increasing in size, 

 slightly embracing, most convex near the umbilicus, and obtusely cari- 

 nated or angular upon the back ; carina more conspicuous near the 

 mouth and gradually becoming obsolete ; umbilicus scarcely as wide 

 as the dorso-ventral' diameter of the outer volution at the aperture, 

 and rather deep; aperture somewhat quadrangular; surface smooth. 

 Diameter, 0.07 inch ; convexity, 0.04 inch. 



Prof. Meek suggests that this species should be placed in the genus 

 Cyrtolites, for, he says, "I know of no other reasons than their small 

 size, smooth surface, and perhaps their less acutely angular dorsal mar- 

 gin, for separating them from that genus." These reasons seem to me 

 sufficient for retaining the name of Hall's genus, Microceras. There is 

 u wide gap between such minute, smooth fossils and the large, sculp- 

 tured CijrtoUtes. 



It is found in the run at Plainville, on Vine street hill, at Versailles, 

 Indiana, and generally at all elevations throughout the Cincinnati 

 Group. It is not, however, very abundant at any locality. Those 

 found by me, at Versailles, are about four times as large as those 

 found at Plainville. 



Genus Cyclora — (Hall). 



Shell minute, smooth, consisting of few whorls; aperture circular. 

 Prof Meek suggests that this genus probably belongs to the fiimily 

 Rissoidoi. 



Cyclora m inula- — (Hall). 



Shell small, smooth, wider than high ; volutions about three, 

 rounded, increasing rapidly in size, so that the last one forms the 

 larger bulk of the shell ; spire dej)ressed ; suture deep, almost chan- 

 neled ; aperture circular, lip thin ; umbilicus small. 



This species has a range, from low water-mark in the Ohio river, to 

 the Upper Silurian rocks, and is dispersed almost throughout the Cin- 

 cinnati Group, It varies in size in different localities. In some localities 

 it is so minute as to appear a mere speck, without the aid of a glass, 



