LOWER HELDERBERG ROCKS. 
185 
This species, so far as known, presents but little variety in its external characters. 
All the specimens examined show a depressed convex ventral valve, while the 
dorsal valve is quite flat. The striae are fine, equal or subequal, gently undulating, 
and increasing both by bifurcation and interstitial addition. A very slight exfoliation 
obliterates the concentric striae. The surface, when very slightly weathered, presents 
numerous punctures arranged in lines parallel with the striae; and these punctures 
are often visible upon the fresh unworn surface, somewhat like the openings in the 
striae of several species of Orthis. 
In the general form and flatness of its valves, this species resembles the S. beckii; 
but the striae are much finer, the width proportionally greater, and the concentric 
wrinkles scarcely conspicuous. It is often associated with the preceding species, from 
which it is readily distinguished. It is more common, however, in the higher beds 
of the Pentamerus limestone. 
Fig. 9 a. An individual of medium size, with the hinge extremities salient. 
Fig. 9 b. Profile view of the same. 
Fig. 9 c. Enlargement of the surface, showing the character of the radiating and concentric 
striae. 
Fig. 10. An individual showing some faint concentric undulations. 
Fig. 11 a. A large individual nearly entire, having the exterior surface slightly weathered, 
and showing the punctae along the striae. 
Fig. 11 b. Profile of the preceding specimen. 
Fig. 11 c. Enlargement of the surface, showing the character of the striae and arrangement 
of the punctae. 
Fig. 12. An individual in which the shell is partially removed, showing obscurely the form 
of the vascular impressions. 
Geological position and locality. In the Pentamerus limestone of the Lower Hel- 
derberg group : Helderberg mountains; Schoharie, and Dryliill, town of Litchfield, 
Herkimer county. 
Strophodonta lieadleyana (n. s.). 
Plate XX. Fig. 1, 2 & 3. 
Shell nearly semicircular, about three-fourths as long as broad. Ventral 
valve very concave especially near the front, sometimes depressed 
convex at the beak : beak scarcely elevated above the margins of the 
area. Dorsal valve depressed at the umbo, and very convex towards 
the front : beak not extending beyond the cardinal margin. Hinge line 
equalling the greatest width of the shell, crenulated. Area somewhat 
[ Paleontology III.] 24 
