PR0DUCTELLH3 OF THE CHEMUNG GROUP. 
1179 
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 6 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. 
