STROPHODONTJE OP THE UPPER HELDERBERG GROUP. 9J 



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. cavumhona, and S. punctulifera 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. 



4 



The figures 1 a-i on Plate xiv illustrate the gradations in size, from a specimen three- 

 fourths of an inch to one of two inches and a quarter in length. 



Figure 1 i 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-hoUow ; Clarence-hollow : Wil, 

 liamsville ; and in all localities of the limestone. 



