S«0 PALAEONTOLOGY OF NEW-YORK. 



Fig. 8 a. View of the aperture and lower side of the shell (which is represented as too 

 smooth in the figure, from a little adhering stone). 



Fig. 8 6. View of the spire and upper side of the volutions, showing the folds or carina. 



Fig. 8 c. An oblique view, showing the sinuosity in the anterior margin of the aperture. 



Fig. 9 a. An older specimen in which the inequalities from the upper part of the last volu- 

 tion are worn off, the plications being more strongly marked on the peristome. 



Fig. 9 b. Dorsal or profile view, showing the outer volution above the apex of the spire. 

 The deep sinus in the anterior margin of the younger shell is not conspicuous in 

 this one, the peristome having become strongly and almost regularly plicate, 

 except on the posterior side, where there is a broad shallow sinus. 



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

 group ; Albany and Sclioharie counties. 



Platjceras retrorsuni ( n. s.). 



Platk LVIII, Fio. 10 a 6, & 6 c; and Plate LIX. Fio. 9 a, b. 



Shell spiral ; the apex above, or on the same plane with the outer volu- 

 tion : volutions about two, the first one nearly smooth, gradually 

 becoming spirally angulate or plicate upon the surface ; upper side 

 flattened, the lower side rounded : aperture somewhat round ; peri- 

 stome sinuate. 



Surface marked by fine crowded transverse striae which are extremely 

 sinuate, the sinyosities varying at different stages of growth, crossed 

 by fine revolving striae. 



This species resembles the preceding in some respects; but in three specimens 

 examined, the volutions are free, the shell a little more robust, and some of the 

 angles more sharply defined. One specimen, which may be a variety of the same 

 species, is less angular, more extended on the umbilical side, and has the apex of 

 the spire below the plane of the outer volution. 



Fig. 10 a. The upper side of the spire, from which the shell is worn smooth and partially 

 removed. 



Fig. 10 b. Profile view, showing the dorsal side and the elevated apex of the spire. 



Fig. 6 c An imperfect specimen, which may be the young of the same. 



Plate LIX. Fig, 9 a, 6, A specimen which is probably of this species. The specimen figured 

 is broken at the apex, and somewhat otherwise distorted. 



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

 mountains : Albany and Schoharie counties. 



