I was walking the beach and noticed this shell. This is the inside, so you're looking at a concave surface. It was different from all the other shells because it had this intricate pattern on the inside surface. So I got out my 'sea life' guide, and I think it is a Green False Tinkle. (What a weird name). I went looking for more, and soon discovered that the main bivalve mollusk on this beach is a 'Butter Clam', and only about 1 in 1000 are this False Tinkle. I found about a dozen more over the next few hours, but the first one (this one) was the best.
David