This picture shows what appears to be a Homilodonta filistriata clam or pelecypod fossil. It was found in the Kope Formation (Eden Group) of Carroll County, Kentucky, USA. It dates to the Ordovician Period. According to the fossilworks web site, it is also known as Ctenodonta filistriata and Deceptrix filistriata.
It is still in matrix and appears to have both valves.