Organization and mode of secretion of the granular prismatic microstructure of Entodesma navicula (Bivalvia: Mollusca)

The term homogeneous has been applied to molluscan microstructures that lack a readily discernible microstructure and as a result, it has become rather a ‘dustbin’ term, covering a multitude of unrelated finely crystalline textures. Here we investigate in detail the outer ‘homogeneous’ layer of the lyonsiid bivalve Entodesma navicula. The apparently equigranular crystals (up to 10 µm) are, in fact, short prisms which grow in a dense organic matrix with their c‐axes and fibre axes coincident, perpendicular to the growth surface. These prisms are distinct from the aragonitic prisms grown by other bivalves in both their morphology and their mode of growth and so we propose the term granular prismatic microstructure. The organic content of granular prisms (7.4%) is the highest yet recorded for any molluscan microstructure and it is apparent that the short prisms have grown within a gel‐filled space. Although this high organic content is likely to make the microstructure metabolically expensive to produce, it has the benefit of making the valves very flexible. This may be advantageous in the intertidal zone inhabited by E. navicula by allowing a tight seal between the valves.