I am not an expert on inverts, but I am somewhat familiar with the James City Formation from my collecting time in the PCS Mine in Aurora. <i>Mercenaria</i> is quite common along with a lot of other shelly material and a few great whites. It was not the prized hunting material for those of us interested in mammal and pliocene sharks, so I did not spend a lot of time there. Linda McCall is the one that you need to contact, though, since she is very knowledgable about invertebrates. I am sure that she could help you. I’ll be interested to read what she posts.