We always called this long thin shell (pictured on top of two *Pen Shells) a “fingernail”. The correct name for it is a Jackknife Clam.
I believe this one might be the “Minor Jackknife Clam” since it was found in Florida and the Atlantic Jackknife Clam is larger and common in New England where it is sold in local markets to eat.
The Minor Jackknife is found from New Jersey to Florida and Texas.
*Information taken from the National Audubon Society’s Field Guide to Shells.