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A new genus and species of mytilid (bivalve) molluscs has been discovered living within shallow water fungid corals in the Gulf of Eilat at the northern end of the Red Sea. Specimens referred to the forma gardineri have been found in deeper water fungids in the Maldives. The animals live ventral side uppermost in cavities excavated chemically by the mantle tissues (pallial envelope) which envelop the delicate shell. The long siphons open into the coelenteron where food, consisting probably to a significant extent of symbiotic zooxanthellae discharged from the tissues of the coral, is collected. Feeding and digestive organs are well developed, labial palps are vestigial. The animal is dorsoventrally flattened with both adductors greatly reduced, the anterior one, with the mouth, being displaced posteriorly. The mantle cavity has a unique unpaired extension (accessory mantle cavity) which extends forward, dorsally and then posteriorly. It may assist in the distribution of hydro-static pressure and so in the even application of the pallial envelope against the wall of the cavity. The byssiferous foot is relatively large and active with the pedal ganglia forming the major nerve centres in the body. The evolution of the genus is discussed. It is basically heteromyarian and has evolved along lines totally distinct from those taken by the rock-boring (but also mytilid) Lithophaginae. The life history is unknown but must involve initial penetration of the coral tissues by way of the stomadaeum or coelenteron.