LOWER HELDERBERG ROCKS, 313 



PLATE LVr. 

 Fig. 5 a. View of the spire of a specimen of ordinary size. The striae are not sufficiently 

 undulated in the drawing. 



Fig. 5 b. 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 &long the summit, and the abrupt bending of the striae. 



Fig. 6 6. A still farther enlargement, showing the fine revolving striaa 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 stria9 has been traced 

 nearly to the aperture, where it suddenly ceases, and the shell is without sinus 

 on ks 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 Shaly limestone, and in 

 the compact layers of the same : Helderberg mountains, Schoharie, Carlisle, Cats- 

 kill and Becraft's mountain. 



Platyceras robustuni ( n. s.). 



Platk LV. Fig. 10 & 11 a, b. 



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 or 

 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 



' [ Paljeontolooy III.] 40 



