LOWER HELDERBERG ROCKS. 331 



Platjceras retrorsuiii, var. al)iioniiis. . 



Plate LX. Fig. 4 a, b, c. 



This shell resembles the P. retrorsum, except that it is more depressed on the 

 upper side, and the aperture transverse. The lower side of the last volution is flat- 

 tened and produced, the peristome on that side extending in a broad linguiform 

 process much beyond the upper or anterior side, leaving a deep sinus upon the back 

 of the shell. This peculiarity can scarcely be regarded as of specific importance. 

 The upper side is plicate, and transversely marked by lamellose striae. 



Fig. 4 a. Upper side of the shell, showing the plications and the strong lamellose striae. 

 Fig. 4 6. The lower side of the same, which is marked only by concentric lamellose striae. 

 Fig. 4 f . An enlargement of the striae on the upper side. 



Geological position and locality. In the middle part of the Lower Helderbei^ 

 group : Schoharie county. 



Plat) eeras iiiteriiiediuui ( n. s.). 



Plate LVIIL Fia. 11. 



Shell arcuate subspiral, making a little more than two volutions, the last 

 one free and becoming gradually straight. The apex is very minute, 

 consisting of little more than a single volution with the parts closely 

 contiguous. 



ScBFACE marked by fine lamellose transverse striaB, which are slightly 

 undulated upon the body of the shell. 



The specimen presents a few slight undulations upon the surface, but is not 

 ridged or plicate like the two preceding ones. 



Fig. 11. View of the upper side of the spire, which is incorrectly represented at the apex. 

 The apparent contraction toward the anterior side of the aperture is due to acci-> 

 dent. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Schohal"ie county. 



Figures 12 a, b, are casts of a species having free volutions, probably of the 

 preceding or of P. retrorsum, from the shaly limestone near Catskill. 

 [ Pal^kontology III.] 41 



