The fossil appears to be Cyprimeria alta pelecypod fossil. It existed during the Cretaceous Period (about 70 million years ago). This fossil was found in McNairy County, Tennessee (Ripley Formation).
Fossils from this locality have an identification PDF at this LINK.
Learn more about these fossils at the Coon Creek Science Center web site: http://www.memphismuseums.org/coon_creek-overview
Fossil from the Herb Miracle collection.