Obviously, the pearl oysters have nacreous shell interiors, including
Pulvinitidae which has a single living species.  Most nuculids have
nacreous interiors (except for Condylonucula), as do most anomalodesmatans,
including Pholadomya candida, the only living representative of
Pholadomyidae, and the extinct Margaritaridae, with a single genus.

However, there are lots of things with non-nacreous shell interiors,
including modern veneroids, caenogastropods, euthyneuran gastropods,
arcoids, pectinoids, ostreoids, limoids, solemyoids, and nuculanoideans.

David Campbell

"Old Seashells"

Department of Geological Sciences
CB 3315 Mitchell Hall
University of North Carolina at Chapel Hill
Chapel Hill NC 27599-3315
USA

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919-962-0685
FAX 919-966-4519

"He had discovered an unknown bivalve, forming a new genus"-E. A. Poe, The
Gold Bug