PRODUCTELL^ OF THE CHEMUNG GROUP. 179 



Dorsal valve moderately or deeply concave, often nearly flat in the 



upper part, and more abruptly deflected towards the front. 

 Surface of the ventral valve, near the cardinal angles, marked by a few 

 comparatively strong wrinkles, which are sometimes continued across 

 the body of the shell, or appear as imbricating lines with finer inter- 

 mediate striae. These wrinkles are studded with very small slender 

 spines, giving a row along the hinge-margin, and extending in nume- 

 rous concentric lines over the entire surface of the shell. The inner 

 surface of the dorsal valve is marked by concentric wrinkles and 

 numerous ranges of pustules; but no spines have been observed. 

 The cast of the ventral valve shows a very narrow area, with a minute 

 central callosity and a bilobed apex, beneath which are cavities from the 

 removal of the teeth. The imprints of the occlusor muscles are narrow 

 elongate scars, and the wide divaricator muscular imprints are rarely 

 faintly preserved. 



- The spine-bases are seen in the casts as minute pustulose elevations, 

 which become more elongate towards the front of the shell. 



The impressions of the exterior of the dorsal valve frequently, show a 

 truncation of the apex (more conspicuously than some of the ' ventral 

 valves, in which the truncation is beneath the arch of the umbo) ; but 

 this feature does not appear to be constant. There is a pretty strong 

 bilobed cardinal process, with teeth sockets on each side ; but no median 

 septum has been observed. The cardinal process, in its exterior impres- 

 sion, shows that each division is grooved, or again lobed. The interior 

 of the valve is minutely papillose, the papillae becoming elongated 

 towards the front of the valve. 



The figures illustrate the prevailing form and size of this species, which occurs in great 

 numbers. Figure G is in part a restoration ; giving, however, the ordinary length 

 and proportions of the spines. The interior of the dorsal valve, figure 7, is enlarged 

 two diameters. 

 Geological formations and localities. This species occurs in some hard arena- 

 ceous bed, at Forestville in Chautauque county ; in a friable sandstone at Cone- 

 wango, and in some semicalcareous layers at East-Randolph, Cattaraugus county, 

 New- York. 



