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 lamellse. 
Fig. 8 a. The exterior of the shell, of the natural size. 
Fig. 8 b. 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. 
Coiiocanlium iiiceptum (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 is 
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 Helderberg limestone, 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. 
