STROPHODONTiE OP THE CHEMUNG GROUP. 
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 with 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. nervosa. 
PLATE XIX. 
Strophomena.nervosa : Hall, Geol. Report of 4th District, p. 266, f. 1. 
Strophomena delthyris : Conrad, Jour. 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. 
[ Palaeontology IV.] 
15 
