STROPHODONTiE OF THE UPPER HELDERBERG GROUP. 
95 
width of the shell. The muscular impressions of the dorsal valve are 
narrow and elongate, and are separated by a rounded median ridge which 
divides above and continues in two diverging processes, the full extent of 
which is not shown in any of our specimens. 
This species, of the Upper Helderberg group offers no important differences from 
the S. headleyana of the Lower Helderberg rocks, although that species has not 
been observed in the intermediate formation of the Oriskany sandstone. It may 
be doubtful, moreover, whether a more extensive series of specimens would not 
show that this form, the S. headleyana , S. cavumbona, and S. puncfulifera are all of 
one species presenting variations of size, degree of convexity, width of area, and 
differences of surface markings, due to the influence of the sediment and other 
surrounding conditions which affect the development of animal life. 
In the Lower Helderberg specimens of S. headleyana , so far as observed, the 
concavity of the ventral valve is usually uniform; and only in two specimens from 
the Corniferous limestone have I observed an elevation along the middle, which, 
in one specimen, is raised into an angular ridge. The muscular impressions of the 
specimens in the Corniferous limestone are always proportionally larger than those 
of the Lower Helderberg formation. 
The figures 1 «-ion Plate xivillustrate the gradations in size, from a specimen three- 
fourths of an inch to one of two inches and a quarter in length. 
Figure 1 b is an impression of the dorsal valve, showing the punctate surface and the 
cavities of the cardinal process. 
In figure 1 g the triangular space indicating the usual position of the foramen is diffe¬ 
rently tinted, and the narrow callosity is shown along the middle. The striated or 
crenulated portion of the area is likewise limited, as shown in the figure, though 
less distinctly in the specimen. The inner edge of the area is indented on each side 
of the middle, corresponding to the branches of the cardinal process, as shown in 
figure 1 h, while the callosity beneath the centre protects that portion which is 
more prominent. 
The figure 1 * is a cast from the Schoharie grit. 
Geological formation and locality. In the Schoharie grit, in Albany and Scho¬ 
harie counties; and in the Corniferous limestone in the Helderberg mountains- 
and at Schoharie; at Cherry-valley; Onondaga-hollow; Clarence-hollow: Wil, 
liamsville ; and in all localities of the limestone. 
