Lucina pensylvanica (Linnaeus, 1758) is relatively common on the East Coast of Florida, but is very rare or absent from the shores of Southwest Florida. The shell in this species may reach 2 inches in length. It is circular, with marked, commarginal (“concentric”) ridges and a strong posterior groove (the “cut” across the surface of each valve).
But what is really cool about this species is its periostracum: it consists of delicate, small, blade-like extensions (in red, below), each ending in a calcified (hard) “scale” or "blade." Well-preserved periostraca are difficult to observe, as beach-collected valves are usually damaged and eroded. Curiously enough, this marine clam does not occur anywhere in Pennsylvania.
The illustrations and details above of a paired, complete shell collected by Don Swenson on Coral Cove, Palm Beach County, Florida, show some of the scaly periostracum still left on that shell. I am attaching a PDF of work published by John Taylor of the Natural History Museum in London and collaborators (at the end of this post), where they explain the periostracum of this species in fascinating detail....The two images below, from the work by Taylor et al, show one blade-like extension in cross-section; red is periostracum protein (soft material), and green is calcium carbonate (hard material) The shell photos above were taken by James F. Kelly for the Museum's Digital Imaging Project.