A general investigation of the molluscan shell microstructures within the Bivalvia (Pelecypoda) for the purposes of paleontologic and modern taxonomic studies indicated that it might be possible to differentiate skeletal structures on a lower level, generic, than was heretofore considered feasible. Since the crystals are the products of body chemistry secreted in closed systems, the crystals and (or) structures may mirror the difference in body chemistry between genera and perhaps species.

The foliate structure was chosen, and the genera Crassostrea, Anomia, Placopecten, and Aequipecten were compared by means of thin sections, peels, and a new technique with single crystals. The shells are disaggregated by removal of the binding organic matrix with EDA reflux and separation in an ultrasonic bath. All material was checked with the optical microscope and studied by carbon-platinum replicas and solid dispersions with Phillips EM 75C and EM 100B electron microscopes.

The foliate structure is produced by the aggregation in layers called folia of calcite crystals elongated along one of the a axes and flattened at the c. One parallel set of m and the 2c faces are dominant, resulting in lath-shape crystals. The c axis is normal to and the a parallel with the flattened folia surface and each of the crystals within one folia is oriented in the same sense.

In the disaggregate preparations, the crystals of Anomia, which are wider, longer, curved along the developed a axis and taper toward the ends, are easily recognized in the light microscope and differ markedly from Crassostrea's needle-like, short, straight crystals whose ends appear blunt. The crystals of the pectinid genera, though differing from both Crassostrea and Anomia, are similar in appearance and on the criteria used to date can not be differentiated.

The Carbon-Platinum replicas reveal surface features on the crystal surfaces, growth bands, parallel striations, etc. which may, with further investigation, prove of taxonomic value.

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