I can't think of many marine bivalves which have much in the way of pattern in the periostracum, but there are certainly plenty of marine bivalves which have a stark white shell under a dark periostracum. A few that come to mind are Nuculidae, Nuculanidae, Arcticidae, Astartidae, Mactridae, and Mytilidae. In the last-named family, some species have a dark colored shell under a dark periostracum, while others have a white shell. Paul M.