LOWER HELDERBERG ROCKS. a»r 



below, marked on the lower side by a sharply defined spiral groove, 

 and on the upper side by a broader depression having a narrow rounded 

 dorsal lobe and a subangular ridge above ; while below, or within the 

 narrow groove on the lower side, is a less strongly defined parallel 

 ridge : aperture subquadrangular ; peristome sinuate. 

 Surface marked by transverse undulating ridges, and by fine longitudinal 

 abruptly undulating striae, which, under a lens, appear to be crenulate. 



This small specimen appears to be a mature shell, and possesses characteristics 

 not observed in any other species. It bears a remote resemblance to P. trilobatmn ; 

 but the volutions are fewer, that one having more than two contiguous volutions, 

 and, when of the size of the specimen here figured, having a very different form. 



Fig. 1 a. View of the upper side of the spire. 



Fig. 1 6. The lower side, showing the narrow groove in the last volution, 



Geological ■position and locality. In the upper part of the shaly lin^estoue of the 

 Lower Helderberg group : Becraft's mountain, Columbia county. 



The following species are from the Upper Pentamerus limestone, and 

 have not been observed in the Shaly limestone below. 



Platjceras claiatuiii ( n. s.). 



Plate LXV. Fig. 2. 



Shell spiral ; the apex consisting of about two symmetrical contiguous 

 volutions, which are gradually expanding : below the second volution 

 the shell is expanded somewhat abruptly, swelling out in an ovoid 

 form : aperture elongate-ovate ; peristome sinuous. 



Surface marked by fine undulating lamellose transverse striae, which, in 

 well-preserved specimens, are crossed by longitudinal undulating strisB, 



This species, in the extreme length of the aperture and form of the body-volution, 

 differs from any other in this formation.' 



Fig. 2. A specimen of this species, from which the shell is nearly exfoliated. 



Geological position and locality. In the upper pentamerus limestone : Schoharie 

 county. 



[ PALiBONTOLOOY III.] 43 



