PRODUCTELL^ OF THE CHEMUNG GROUP. 185 



The occlusor muscular impressions are well defined, semielliptical 

 and of moderate breadth, appearing to be duplicate ; the surface imme- 

 diately below them is transversely corrugated over a small oval space. 

 The cardinal muscular impressions are wide and comparatively short, 

 flabelliform and deeply striated. The cast of the rostral cavity is grooved 

 on the upper side, and its extremity simple ; showing on each side, be- 

 neath the apex, cavities made by the removal of the dental » lamellge, 

 while the cardinal line presents no evidence of an area beyond the thick- 

 ness of the shell. 



The interior of the dorsal valve is papillose above, becoming papillose- 

 striate towards the margin. There is a slender mesial septum, which 

 becomes stronger above, and terminates in a short cardinal process : on 

 each side of this are slightly oblique socket-plates, and between these 

 and the hinge-line are narrow teeth-sockets. The cardinal process is 

 distinctly bilobed, and each division is slightly divided or grooved at 

 the extremity. The occlusor muscular impressions are small oval-ovate 

 spots, but are not well-preserved in any specimen seen, yet the reniform 

 vascular impressions are sometimes strongly marked. In addition to the' 

 ordinary vascular impression, which terminates abruptly at some dis- 

 tance from the median septum, there is sometimes another mark of 

 similar character beginning at a little distance within the termination 

 of the reniform impression, and curving forward to the termination of 

 the median septum, and thence recurving it terminates on the other 

 side in the same manner as it began, describing part of an elliptic curve. 

 This shell has a very symmetrical form, and is the largest among the species 

 of this subgenus. It is readily distinguished from all others by its regular wide 

 form and equal convexity, as well as by its distantly disposed and very slender 

 spines, by the peculiar punctate structure of the exfoliated shell, and the striato- 

 punctate surface of the cast. 



The form and character of well-preserved specimens are shown in figures 36, 37 and 38, 



which give the outline, cardinal and profile views of a ventral valve. Figures 39 and 40 



are cardinal views of the casts of ventral valves, showing the muscular impressions, and 



one of them showing on each side of the beak th6 cavities made by the dental lamella). 



Figures 30 and 31 are of the interior of two dorsal valves ; and figure 32 is a cardinal 



view, showing the contour of the interior. 



Geological formation and localities. This species occurs in the arenaceous and 



semicalcareous beds at Conewango, at Napoh, and near New-Albion, Cattaraugus 



county. 



[ Pal^ontolooy IV.] 24 



