SPIRIFERiE OF THE UPPER IIELDERBERG GROUP. 
191 
Doesal 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 elevated, area linear. 
Sueface 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. Tli3 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. mucronatct , but the plications are a 
little more prominent, and the concentric stria? 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, were casts of 
small and poorly preserved individuals ; but a larger and more extended collec¬ 
tion has shown 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 are usually very imperfect and unsatis¬ 
factory. The illustrations figures 22, 23 and 24, are from some of the better pre¬ 
served specimens. 
Geological formations and localities. This species occurs in the Schoharie grit 
in the Helclerberg mountains, Albany county, and at Schoharie; and rarely in 
the Corniferous limestone in tho eastern part of the State. It is not uncommon in 
the Corniferous limestone at Williamsville and other places in the western part 
of the State, and I have a single specimen from Ohio. Its associates in the Scho¬ 
harie grit are the S. duodenaria and S. raricosta . 
