These fossils appear to be Crassatella vadosus pelecypods (named by Morton 1834). They existed during the Cretaceous Period (about 70 million years ago). This fossil was found in McNairy County, Tennessee (Ripley Formation).
See their 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
Fossils from the Herb Miracle collection.
