FOSSILS OF THE CLINTON GROUP. 551 



man's Quarry, and in the corresponding formation at Geo. Young's 

 Quarry. It also occurs in the upper shaly courses at Huffman's, and 

 in the upper ferriferous courses at Todd's Fork, Ohio. In the Clinton 

 on the east side of Taylor's Ridge, Summerville, Georgia. 



Cyclonema bilix, Conrad. ... ,-, 



( Plate 26, Fig. 15, var ; Plate 30, Fig. 15 ; Plate 37A, Fig. 9, var.) 



The outer lip of the aperture is sharp. The inner lip is strongly 

 broadened, thus increasing a little the size of the aperture. This broad- 

 ened flange of the lip meets the lower surface of the shell with a concave 

 curvature, which is strongest toward its inner end of attachment to the 

 previous, whorl; here it forms often a strong though rounded groove 

 with the slightly raised margin of the shell. In other respects the shell 

 is very variable. In the ordinary form the whorls are rounded. Very 

 distinct fine close sharp striae of growth traverse the shell transversely, 

 indicating the outlines of the aperture at various stages of development. 

 In addition to these striae, transverse wrinkles of moderate distinctness 

 and having the same general direction, also are frequently present. The 

 longitudinal revolving striae are much less closely placed, and at more or 

 less regular intervals certain of these striae are very strong and sharply 

 elevated, forming the most prominent lines in the exterior ornamenta- 

 tion of the shell. This form is found at Soldiers' Home, Fauver's, Huff- 

 man's, Centreville, Brown's, Todd's Fork, Ohio, and Hanover, Indiana. 

 (Plate 30, Fig. 15.) 



In a variation of this form the surface ornamentation is iden- 

 tical, but the spire of the shell is much more strongly elevated, 

 the apical angle is therefore less and the successive whorls 

 do not differ so much from each other in transverse measure- 

 ment. Succeeding whorls do not conceal quite so much of 

 the earlier smaller ones, and the latter are therefore more ex- 

 posed and appear more convex. This form occurs at Brown's Qaurry. 

 A third variation shows a similar increase in height of the spire, and a 

 corresponding rounding of the successive whorls. But in this case the 

 last or fifth whorl shows only the fine transverse striae; these are less 

 sharp and hence give a more fibrous appearance to the ornamentation. 

 The longitudinal revolving striae are either absent upon the main surface 

 of the whorl, reappearing only towards the lower side of the shell, or else 

 they are at least sufficiently indistinct not to be readily seen at an ordi- 

 nary examination of the shell. The younger whorls however usually 

 show these revolving striae more distinctly, indicating that this is a charac- 

 ter lost only in the adult shell, while still present in younger specimens. 

 This form is found at the Soldiers' Home. In it the transverse wrinkles 

 are more prominently developed, especially along the suture line of the 

 last whorl. 



