Monograph of the Gasteropoda. 309 



three (rarely more than two visible), rapidly enlarging toward the 

 aperture ; shell sharply and strongly carinated upon the back, and 

 obtusely angnlated upon the sides ; ventral side obtusely angulated, 

 Avith a narrow, deep groove on the summit, for the reception of the 

 dorsal carina ; aperture but slightly expanded, quadrangular ; section 

 quadrangular ; dorsal slopes marked by strong, obliquely transverse 

 ridges, which extend to the angle on the side of the volution ; entire 

 surfa(3e marked by fine transverse stride, the spaces between which are 

 crossed by finer curving ones, giving the surface a cancellated appear, 

 ance. 



It is the type of the genus Cyrtolites. 



The range of this species i^ co-extensive with the Cincinnati Group, 

 but it usually occurs in the form of mere casts, which do not preserve 

 the surface markings. It is frequently found incrusted with a bryo- 

 zoum so minute as not to destroy the shape of the oblique transverse 

 ridges, which mark the slopes from the dorsal margin to the angles ou 

 the sides of the volution. Well preserved shells are quite rare, though 

 I found good specimens near Versailles, Indiana, about 300 feet below 

 the Upper Silurian rocks. 



Cyrtolites Dyeri — (Hall, 1871). 



Shell small, laterally compressed, consisting of two or more volu- 

 tions, the outer one embracing the inner for about half its breadth, 

 bearing a moderately wide umbilicus, in which may be seen a portion 

 of the preceding volution ; sides of the volution convex, obtusely sub- 

 angular near the margin of the umbilicus, into which it curves ab- 

 ruptly and more gradually declines, with a slight convexity toward the 

 salient subcarinate dorsum. Transverse section, cordiform, broadest 

 near the umbilical margin. 



Surface of the shell marked by from eight to twelve nearly equi- 

 distant revolving ridges, with sometimes smaller intermediate ones, 

 and also crossed by numerous closely arranged transverse lamellose 

 ridges, having a double backward flexure between the revolving lines 

 and a general retral direction toward the keel of the shell. 



Found in the upper part of the Cincinnati Group, east of Maysville, 

 Ky., at Clarksville, in Clinton county, and near Freeport, Ohio, at 

 about the range of the Glyptocrinus Nealli. While its distribution is 

 so general, it is yet a rare fossil, owing probably to its ratlier small 

 size, and, except when the shell is well preserved, inconspicuous ap- 

 pearance. 



