I acquired this shell at a rock/fossil/mineral show this past spring from someone who collected it themselves. It's a Chesapectin jeffersonius pelecypod from the Yorktown formation (Neogene, Pliocene epoch, Zanclean stage) of Virginia and was found along the James River. The shell is enormous and I can just imagine what the scallops would have tasted like! It's only a single valve, I'm not sure if it is the left or the right but was a host to some epibionts. There are large barnacles (Balanus concavus), barnacles spats and some corals (Septastrea marylandica). All in all it's a terrific specimen that gives evidence of the environment that it lived in.
I think these small holes are the traces from the boring sponge Cliona sp.
A better view of the huge barnacles
and some shots of the corals.
Corals with Cliona sp. boring sponge traces