Some bivalves use periostracum structurally.  Diplodonta frequently constructs a nest using periostracum and debris.  Rastafaria has long, tube-like periostracal structures, speculated to provide a conduit for anoxic sulfur-rich water to reach the symbiotic bacteria.  This periostracum is the main feature used to distinguish the genus.

    Dr. David Campbell
    "Old Seashells"
    Biology Department
    Saint Mary's College of Maryland
    18952 E. Fisher Road
    St. Mary's City, MD  20686-3001 USA
    [log in to unmask], 301 862-0372 Fax: 301 862-0996
"Mollusks murmured 'Morning!'.  And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!