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.