These pictures are of a Devonian Period clam called Modiomorpha concentrica. It was found in Clark County, Indiana in the Jeffersonville Limestone. A coral is growing on part of the shell that is probably an Aulocystis.
This fossil belongs to my cousin who has been cleaning it.
I used A Monograph of the Fossil Shells of the Silurian and Devonian Rocks of Kentucky by Henry Nettelroth (1889) found here. This first image is from Plate II figure 10.