322 PALEONTOLOGY OF ILLINOIS. 



Surface of the valves marked by numerous raised lines or costaB, 

 that increase towards the base by implantation, and are about as 

 wide as the sp'aces between them. From 80 to 90 of these costae 

 may be counted on the borders of the shell. 



Greatest width of the valves 4 inches. Length from beak to base 

 8,'',; inches. Length of hinge-line 3| inches. 



Locality and position : Upper beds of the Keokuk limestone, near 

 Warsaw, 111. The author's collection. 



This is a very rare species, only one valve, and part of another, 

 having been found in 25 years collections from this horizon. 



