This little clam (2.2 cm long by 1.2 cm wide) is found in the Marble Hill Bed, Rowland Member, Drakes Formation of Bedford, Trimble County, Kentucky, USA. The outer shell does not have much detail but the inner part has nice hinge teeth. Thanks to Bill Heimbrock of Dry Dredgers for the identification of Cycloconcha milleri?. Read the paper Review of Ordovician Pelecypods by John J. Pojeta, Jr. Document is in the DJVU format so you will need a reader program like the Windows version of WinDjView on CNET download.com: CLICK HERE.
This pelecypod fossil is from the Ordovician Period Katian (Richmondian) Stage. This dates to about 450-445 million years ago.
UPDATE (2020): The Review of Ordovician Pelecypods is now available on PDF format at the site linked above. I added a possible species name for the specimen as well.
The author of the paper has since passed away Dr. John Pojeta Jr. (1935 - 2017). He retired from the U.S. Geological Survey in 1994. See link for memorial:
https://www.aaps.net/John-Pojeta-Jr-Memorial.htm
