186 PALJEONTOLOGY OF NEW- YORK. 



wide, and marked by transverse striae produced by the continuation 

 of the crenulations from the hinge line across its surface. Foramen 

 narrow, closed. 

 Surface marked by coarse sharply elevated strise, which increase chiefly 

 by implantation, and present a peculiar irregularly waved appearance. 



This species bears much resemblance to S. cavumbona; but the striae are unlike, 

 the area is wider, and the form of the callosity closing the foramen is quite different. 

 The specimens in my possession, though mature forms and larger than S. punctuli- 

 fera, are never as abruptly curved as that species. The radiating strise are crossed 

 by fine concentric lines which are visible in the spaces between, while the casts of 

 the strise are often crenulate. The interiors of several ventral valves show some 

 variety in the form of the muscular impressions, though they all present the es- 

 sential features of double broad lobes with plicate or foliate impressions. 



Fig. 1 a. Interior of the ventral valve, where the surface is marked by strong rounded strise, 

 and the muscular area striate. The hinge along its line of junction with the 

 opposite valve presents a linear groove from the termination of the crenulations 

 to the cardinal extremities : crenulations extending little more than one-third 

 the length of the hinge line on either side of the centre. 



Fig. 1 b. Enlargement of a portion of the hinge line and area of fig. 1 a. 



Fig. 2 a, b. Interior of the ventral valve of two specimens which present some variation in 

 the form of the muscular impressions. 



Fig. 2 c. Enlargement of the papillose interior surface. 



Fig. 2 e. Enlargement of the cast of the muscular area. 



Fig. 2 d. Cast of a ventral valve, showing the narrow callosity which fills the foramen. 



Fig. 2f. Enlargement of the punctate casts of striae. 



Fig. 3 a. Exterior of a ventral valve, showing the peculiar interrupted striation. 



Fig. 3 6. Profile showing the concavity of the ventral valve. 



Fig. 3 c. Enlargement of a portion of the striae. 



Fig. 3 d. Interior of a valve having similar characters to fig. 3 a, but more deeply concave. 



Fig. 3 e. Profile of fig. 2 b, showing the convexity of the interior of the valve. 



Geological position and locality. In compact layers of the shaly limestone of the 

 Lower Helderberg group : Helderberg mountains; Schoharie, Hudson, etc. 



