STROPHODONT^ OF THE CHEMUNG GROUP. 113 



above and is truncate at its extremities. Outside of the muscular impres- 

 sion, the surface is punctate or papillose ; and from the commencement 

 of the abrupt curvature, it is marked by strong vascular impressions. 

 The cast of a convex dorsal valve found in the same strata shows the 

 marks of a strong nearly vertical cardinal process, with remains of socket 

 plates and strong impressions of the occlusor muscles. 



Since we know but this species in the Chemung group mth a concave ventral 

 valve, it is inferred that the convex dorsal valve may belong to the same species. 



The specimens are illustrated in figures 7 and 8 of Plate xix. 



Geological formation and locality. In some arenaceous beds of the Chemung 

 group, to the northwest of Elmira. 



Stropliodonta perplana var. neiTOsa. 



PLATE XIX. 



Strophomena nervosa : Hall, Geol. Report of 4th District, p. 266, f. 1. 



Strophomena delthyrit : Conrad, Jonr. Acad. Nat. Sci. Philadelphia, Vol. viii, pa. 258, pi. 14, f. 19. 



Shell very depressed, subplano-convex ; semielliptical, with hinge- 

 extremities often extremely produced. 



Ventral valve very slightly convex ; and dorsal valve barely concave. 



Surface marked by sharp elevated undulating striae, which are irregu- 

 larly bifurcated, and often swelling at unequal intervals into node-like 

 expansions. 



The casts, as far as observed, are undistinguishable in character from 

 -those of S. perplana. 



In describing from the general form of the shell and surface characters, no one 



would suspect this to be of the same species as the regularly striated S. 



perplana of the Hamilton group and Corniferous limestone. A comparison of 



numerous imperfect specimens retaining some portion of the shell, as shown in 



figures 15 and 16, together with casts of the interior of the same, has left no 



doubt as to the propriery of referring them to S. perplana. 



Figure 14 is from the original specimen of S. nervosa, and is a cast of the ventral 



valve in soft olive shale ; while figure 15 is part of a specimen retaining the shell 



essentially entire. 



[ PAL^ONTOLOOy IV.] 15 



