SPIRIFERiE OF THE UPPER HELBERBERG GROUP. 191 



Dorsal valve convex, often somewhat flattened, particularly at the sides, 

 and becoming concave towards the cardinal extremities : mesial fold 

 prominent and strongly defined, often much expanded in front ; beak 

 small, little eleviited, area linear. 

 Surface marked by from ten to twenty slender well-defined plications 

 on each side of the mesial fold and sinus. In the youngest specimens 

 examined, there are no more than six or eight plications on either 

 side. The entire surface is marked by lamellose concentric striae, which 

 at intervals are crowded into imbricating lines of growth. 

 The casts of the ventral valve show the existence of strong short den- 

 tal plates enclosing a comparatively narrow muscular area. In the middle 

 of this area, in well preserved specimens, are two narrow and much 

 elongated occlusor impressions, with a slight crest between ; but these 

 features are often entirely absent or obsolete. Th3 casts of the dorsal 

 valve show large teeth-sockets, and a distinct prominence in the centre 

 for the muscular attachment. 



This species has the general aspect of S. mucronaia, but the plications arc a 

 little more prominent, iuul the concentric striag less strongly imbricating : the 

 principal point of difference, however, is in the elevated and incurved area of the 

 ventral valve. 



The specimens from which the original description was made, wci'c casts of 

 small and poorl't' preserved individuals ; but a larger and more extended collec- 

 tion has siiown all the gradations of form, size and condition illustrated on Plate 

 27, figure 17 being one of the smaller symmetrical specimens. Individuals from 

 the Schoharie grit, and from the limestone, often present considerable differences 

 in the relative convexity of the valves ; and the area, either from compression or 

 other causes, shows some apparent variations in its height, as seen in figures 21 

 and 28, plate 27. In all cases, however, the height of the area is a distinguishing 

 feature. The casts in the Schoharie grit arc usually very imperfect and unsatis- 

 factoiy. The illustrations figures 22, 23 and 24, are from some of the better pre- 

 served si)ecimens. 



Geological formations and localilics. This species occurs in the Schoharie grit 

 ill the Ilelderberg mountains, Albany county, and at Schoharie ; and rarely in 

 Ihc Corniferous limestone in tho eastern part of the State. It is not tmcommon in 

 the (^)niiferous liujesfoue at Wiiliamsville and other places in the wcstei'n part 

 of the State, and 1 have a single specimen from Ohio. Its associates in the Scho- 

 harie grii are the S. duodcnaria and aS'. raricotitd. 



