STROPHODONT^ OF THE UPPER HELDERBERG GROUP. 81 ' 



STEOPHODONT^ OF THE UPPER HELDERBERG GROUP. 



Stropltodonta deniissa. 



PLATES XI & XII. 

 For description and references, see the same species under Hamilton group, p. 101. 



Tffls species, which has been made the type of the genus, first appears in 

 the Schoharie grit, where it is not of unfrequent occurrence. 



The figures 14, 15 and 16 of Plate xi, are illustrations of specimens found in this rock. The specimens 

 usually occur as casts of the interior and impressions of the exterior of the shell. The coarse ma- 

 terial of the rock prevents the preservation of the muscular impressions with that degree of 

 distinctness which we find in the same species in the Hamilton group, where the material is finer. 

 At the same time it is probable that the shell has been thinner in the Schoharie grit than in the 

 higher formations, since the striae are more strongly marked on the casts. 

 Figure 15, Plate li, is a cast of the interior of a ventral valve, and figure 14 shows the character of 



the exterior shell of the same irom a cast made in the natural mould. 

 Figure 16 is a dorsal valve, which is very thick and strong, occurring in a more calcareous portion of 



the rock. 

 Figure 17 is a cast of the interior of a ventral valve, showing the muscular impressions. 

 Commencing in the Schoharie grit, the species occurs in the Corniferous lime- 

 stone in the form and condition represented on Plate xii, figs 1- 5 : it is rarely 

 seen of larger dimensions. It attains its maximum development in the Hamilton 

 group, occurring in great numbers, and sometimes attaining a length of one and a 

 half by one and three-fourths inches in breadth. It continues in the Che- 

 mung group, where, in the arenaceous beds, it exhibits a character similar to 

 specimens from the Schoharie grit. 



Strophodonta alyeata. 



PLATE XI. 

 Strophodonta alveata : Hall, Sixteenth Report on the State Cabinet, p. 36. 1863. 



This species is known only as casts of the interior and impressions of the 

 exterior. In the young shells, the casts of the ventral valve are slightly 

 convex ; while ir older specimens they are flat or slightly concave, with 

 the margin more or less recurved. The form is semi-elliptical, and the 



[Pal^ontoloot IV.] 11 



