Need help in IDing this shell

Over on the bay side of St. Joseph Peninsula St. Park last year (Florida Panhandle), I encountered many of these shells. They are quite pretty but I don't have a comprehensive shell book and have not been able to identify them. They are nondescript white on the outside but butter yellow with rust-colored zigzags on the inside (see attached photo), and each shell is about the size of a penny. Can anyone identify these for me?

Thanks in advance
Dave