ORISKANY SANDSTONE. 451 



these there is a third (usually) smaller depressed plication, with a shal- 

 low sinus below the middle of the shell. On the dorsal valve the two 

 central plications, below the middle, are a little stronger and more 

 widely separated from the lateral ones, rising in front into a slight 

 mesial elevation. When seen from the front, the margin of the shell 

 presents a very distinct sinus, with a corresponding projection of the 

 opposite valve, the entire margin being deeply serrate. The finer surface 

 markings are mostly obliterated, some imbricating lines of growth alone 

 remaining. 



The fimbriated appendage along the hinge-line has been observed in 

 so many specimens, that I can only suppose it to belong to the animal 

 economy. The remains of this appendage are sometimes observed upon 

 the inner margins of the separated valves. 



This species differs from the preceding in the proportionally wider and less sloping 

 cardinal line, and in the less extension of the beak of the ventral valve, wliiA is 

 smaller and more neatly defined; in the fewer plications and comparatively greater 

 convexity of the ventral valve, while the dorsal valve is slightly convex and ab- 

 ruptly inflected at the margin, features not observable in L.Jlabellites. 



In the localities where this species occurs, often in large numbers, few or no 

 specimens of the L. flahellites are found; while in localities where the surfaces of 

 the layers are covered with the latter shell, no specimens of L.fimbriata have been 

 seen. 



Fig. 2 a, b. Dorsal and profile views of a specimen of ordinary size. 



Fig. 2 c. Front view of a specimen a little greater than the ordinary size, and which has the 

 fimbriated expansion extending below the cardinal angles. 



Fig. 2 d. The ventral side of the preceding specimen, enlarged two diameters. 



Fig. 2 e. The interior of the ventral valve, showing the cardinal teeth and the muscular 



impression. 

 Fig. 2 e. The interior of the dorsal valve, shofwirig the muscular impression, the cardinal 



process, the oblique lamella, median septum and bases of the crural processes. 



Fig. 2f. The same enlarged. 



Geological position and locality. In the Oriskany sandstone : Cumberland, Md. 



