LOWER HELDERBERG ROCKS. 309 



forms the Pileopsis vetusta of Sowerby. The species first described by Mr. 

 CoNKAD are the P. dumosum of the Upper Helderberg limestone, the P. 

 ventricosum and P. gebhardii from the Lower Helderberg; the two latter 

 species being figured on Plate lvii and other plates of this volume. In 

 these species the volutions are few, but contiguous ; and the shells occur 

 with a broad expansion of the lip, or terminate in a thin unexpanded 

 margin. The P. dumosum is spiniferous. Some of the species subsequently 

 referred by Mr. Conrad to this genus have the lower volutions free, and 

 consisting of a single turn at the apex of the shell. 



The species which I have grouped under this genus present a great 

 variety of forms, and it might be questioned whether there may not be 

 reason for a farther separation ; but the variations presented in the same 

 species, when examined in a large number of individuals, render it 

 sometimes difficult to draw the line of specific distinction. 



Oe\L'S Pl.ITYCERAS (Conrad, as emended). 



Shells depressed subglobose, subovoid or obliquely subconical. Spire 

 small : volutions few, sometimes free and sometimes contiguous, with- 

 out columella; aperture more or less expanded, often campanulate and 

 sometimes with the lip reflexed ; peristome entire or sinuous. 



Surface striated or cancellated, often spirally ridged or plicate and some- 

 times strongly lamellose transversely, nodose or spiniferous. 



Many of the species show a sinuosity of the striae, indicating a notch 

 in the margin of the aperture during the first stages of growth, and 

 this notch sometimes remains in the mature condition. More frequently, 

 however, the earlier sinus is closed, and, in certain species, the margin 

 continues unbroken, while in a few others this sinus is continued to the 

 margin at the final period of growth ; but more often it becomes closed 

 at some period during the growth of the shell, and another commenced 

 at some other point ; and not seldom two or more are thus begun and 

 continued, while some simply striated species, with a single sinus in 



