The lucinid bivalve Lucina pensylvanica possesses an unusual flexible commarginal shell sculpture formed from calcified periostracal lamellae. The lamellae comprise thick, recurved, periostracal extensions with distal calcified scales. The periostracum is also densely embedded with calcareous granules around 2.0-2.5 μm in diameter and a thin (10 μm) layer of prismatic aragonite covers the ventral face of each lamella. Other species of Lucina in the western Atlantic possess calcified scales but with different morphologies and the continuous commarginal ridges of the eastern Atlantic Lucina adansoni and other African species are similarly constructed and homologous. The periostracal lamellae are a probable apomorphy of the genus Lucina and morphology of the calcified structures provides a set of systematic characters of importance in the discrimination of species.