LOWER HELDERBERG ROCKS. 
si i 
The species which present themselves in this group of strata, and in 
the succeeding Upper Helderberg rocks, maybe conveniently divided into 
two or three groups, presenting certain general characters by which they 
can be distinguished. These distinctions, however, are perhaps not sus¬ 
tained in the more intimate relations of the shells. 
In the grouping of the species in their succession in the several plates, I have 
endeavored to bring together in some degree the forms more nearly related; begin¬ 
ning with those resembling Platyostoma in form, and progressing to the opposite 
extreme with as much regard to systematic arrangement as was possible under the 
circumstances in which the materials were collected, and the long intervals which 
elapsed between the completion of the earlier and the later parts of this portion 
of the work, and with constantly increasing collections. 
Platyceras ventricosuiii. 
Plate LYI. Fig. 1 - 4 & 8 ; and Plate LVII. Fig. 4. 
Platyceras venlricosum : Conrad, Annual Report on the Palaeontology of New-York, 1840, p. 206. 
“ Shell ventricose ; aperture very large and campanulate; volutions 
“ three, contiguous, depressed below the upper margin of the whorl.” 
The shell is obliquely ovate, spreading rapidly from the apex, and 
becoming extremely ventricose below; aperture campanulate, the lip 
in contact with the spire, and sometimes strongly reflexed. 
Surface marked by fine transverse or concentric lamellose strife, which 
are somewhat undulated and rarely finely cancellated by faint revolving 
striae. 
This species is a comparatively common form in the upper part of the Shaly 
limestone, its usual size being that of the specimens figured on Plate lvi ; rarely 
equalling the size of fig. 4, Plate lviij while I have one specimen which measures, 
from the anterior margin of the aperture across the volutions to the posterior side, 
more than three inches. 
PLATE LVI. 
Fig. 1 a , b. Views of the upper and lower side of a young specimen which is a cast. 
Fig. 2 a. View of a specimen of medium size, looking upon the top of the spire. 
Fig. 2 b. View of the aperture of the same, showing also that the spire is not as high as the 
outer volution. 
