LOWER HELDERBERG ROCKS. 
30S> 
forms the Pileopsis vetusta of Sowerby. The species first described bj Mr. 
Conrad 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. 
Genus PlatYCEIUS ( 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 strim, 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 
