Arcinella cornuta
This bivalve is small, growing to about 1”. Its off-white shell is thick. It has distinct erect tubular spines in 7 to 9 rows across the shell. The interior is shiny and tinted pink to purple. The spikes and the hard shell help protect the Spiny Jewelbox from predators. Specimens found on the beach may have had their spines somewhat eroded.
The Spiny Jewelbox is a filter feeder that extracts plankton from the seawater. The animal starts life cemented to a hard structure. While still young, they detach themselves, then move freely and mature on the sandy sea floor.
Live shells should never be taken from any Florida State Park.