ADDENDA. 491 



somewhat flattened. The muscular impression is double, and situated 

 on each side of a mesial ridge, which becomes thickened and expanded 

 towards the apex. 



This species differs from the preceding in the terminal apex, and in the stronger 

 concentric lamellae. 



Fig. 8 a. The exterior of the shell, of the natural size. 



Fig. 8 6. Two valves in conjunction. It is not determined that these valves are in their 

 natural relations, and no articulating processes have been observed. 



Fig. 8 c. The interior of another specimen. 



Fig. 8 d. The same enlarged. 



Geological position and locality. In the Oriskany sandstone : Cumberland, Md. 



Conocai'diiiiii inceptimi ( n. s.). 



Shell obliquely subovate ; posterior extremity subobtuse ; beaks sub-' 

 anterior. Anterior end short, rounded ; the hiatus rounded in front and 

 narrowed behind, reaching about half the length of the base. Surface 

 of the posterior portion marked by five or more strong ribs on each 

 valve. The anterior parts of the valves are marked by radiating striae, 

 which are likewise more faintly shown on the ribs of the posterior part 

 of the shell. The whole surface marked by concentric lamellose striae, 

 which are much stronger on the anterior part of the shell, and, at the 

 crossings of the radiating striae, marked by little granules or spinules, 

 and the continuation of these into the hiatus gives a denticular cha- 

 racter to the margins. 



I have seen but a single distorted specimen of the species, and the hinge-line ia 

 obscured by adhering stone. It is the only specimen of the genus that I have yet 

 observed in any of the rocks below the Upper Heldorberg limo stone, and this one 

 was not observed till after the plates of this part of the volume had been com- 

 pleted. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Albany county. 



