154 PALAEONTOLOGY OF ILLINOIS. 



curving 1 regularly upward to the dorsal margin; posteflfor 

 terminating in an obtuse point, the ventral margin being more 

 strongly curved at both extremities than the dorsal. The sur- 

 face is covered with round pits which are most numerous and 

 conspicuous near the margins, with traces of delicate longitu- 

 dinal striae along the center of the valve when viewed under a 

 good lens. 



Length 14 / 10 inch; height, 4 /i inch. 



Position .and locality: St. Louis limestone, South St. Louis, 

 Mo. 



Collector, A. H. Wortheu. 



No. 2497 of the Illinois State Museum. 



