The mytilid bivalve Botula cordata (LAMARCK) from the Upper Eocene of the Venetian region, northern Italy, bored into live soft bottom corals, which reacted by forming a calcareous interseptal filling around the borehole. A chemical etching mechanism is inferred from the presence of"etch marks" on the borehole walls. The association with free-living corals capable of counteracting burial by sedimentation or accidental overturning enhanced the survival potential of the bivalve on the soft bottoms. Commensalism with soft bottom corals appears to have evolved convergently at least three times within the Mytilidae, but is unknown among other bivalves.