When I was a kid, we vacationed on Sanibel Island in Florida and I was an avid sea shell collector. One of the most common sea shells I remember was a huge brown fan shaped bi-valve called a Pen Shell (also called Penn Shell but never Pin Shell).
After a storm, the beaches would be covered with them.
They were usually about 8-10 inches long and a glisteny iridescent brown, especially shiny on the inside.
Pen Shells are are part of the family Pinnidae which are large saltwater clams.
Pen Shells are used in inlay and as heishi.
Close up of Pen Shell heishi
Pen Shell heishi used on a fetish necklace
Turquoise nugget necklace with Pen Shell heishi spacers
Pen Shell is sometimes used in inlays and especially since tortoise shell was banned in 1973.