LOWER HELDERBERG ROCKS. 183 



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 pai'allel 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 iudividual 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 fl. A large individual nearly entire, having the exterior surface slightly weathered, 

 and showing the punctaa along the striae. 



Fig. 11 6. Profile of the preceding specimen. 



Fig. 11 c. Enlargement of the surface, showing the character of the striae and arrangement 

 of the punctac. 



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 Dryhill, town of Litchfield, 

 Herkimer county. 



8troplio(lonta Iieadleyana ( 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 Avidth of the shell, crenulated. Area somewhat 

 [ Pal^ontolooy III.] 24 



