LOWER HELDERBERG ROCKS, 
313 
PLATE LVI. 
Fig. 5 a. View of the spire of a specimen of ordinary size. The striae are pot sufficiently 
undulated in the drawing. 
Fig. 5 5. Profile of the same from the back of the shell. 
Fig. 6. The spire of another specimen where the last volution is more free near the aperture, 
and slightly carinated near the apex. 
Fig. 7. The cast of a specimen of this species. 
Fig. 9. A similar cast, the last volution becoming free, 
PLATE LV. 
Fig. 6 a. An enlargement of the spire of the specimen fig. 5, Plate lvi ; showing the ca- 
rination along the summit, and the abrupt bending of the striae. 
Fig. 6 5. A still farther enlargement, showing the fine revolving striae which cancellate the 
coarser transverse striae, and the proportionate size of the two sets. In another 
specimen, this carinated line and abrupt bending of the striae has been traced 
nearly to the aperture, where it suddenly ceases, and the shell is without sinus 
on its margin. 
For farther observations on this and the preceding species, see remarks under 
head of Oriskany sandstone. 
Geological position and locality. In the upper part of the §lialy limestone, and in 
the compact layers of the same : Helderberg mountains, Schoharie, Carlisle, Cats- 
kill and Becraft’s mountain. 
Platyceras rolmstum (n.s.). 
Plate LY. Fig. 10 & 11 a, 5. 
Shell robust, somewhat ovoid, very slightly oblique, gradually expanding 
towards the aperture which is campanulate, and transversely broad- 
oval : volutions about three, the inner ones small; the outer volution 
thick and strong, somewhat regularly rounded. 
Surface marked by undulating lamellose striae, which are in some parts 
crowded into elevated folds or wrinkles, with a distinct double sinuo¬ 
sity on the right side of the aperture, and a broader one on the left op 
umbilical side. Fine revolving striae are sometimes observed cancellating 
the transverse striae. 
This species presents a general similarity to P. ventricosum in the depression of 
the first volutions of the spire below the plane of the outer ones, but the specimens 
are never so rapidly expanding or ventricose; and the last volution, at its junction 
with the labrum, is, in this species, much broader and stronger; while the volutions 
[ Paleontology III.] 40 
