I found the large clam shell on the left on Captiva Island near the Mucky Duck Restaurant at the end of August. Nearby are coquina shells. I looked through a seashell guidebook called Florida's Seashells, A Beachcomber's Guide and was unable to identify it. I posted this picture on the I Love Shelling Facebook page and didn't get a response. Hmmm... if anyone knows what it is, please leave a comment.