Pen Shells

I have always called these shells sea fans or fan shells, but they are called pen shells. Pen shells are actually a type of clam and they live in soft and sandy mud.
This is usually about the time of year you find tons of these on the beach, but, so far, I'm only finding the normal amount.
However, in all the years that I have seen pen shells on the beach, I've never seen one closed with the clam still inside. The first picture is a typical pen shell picture with rough spines along the shell. The inside of this shell is creamy with a beautiful iridescent purple. The second picture is of an intact pen shell with clam visible inside(click on the picture to enlarge and you can see the clam easily.)
Pen shells have a tiny foot which sends out "threads" to wrap around buried shells and rocks. The threads, called byssus, are so strong that it is said they were used in woven garments for Roman Emperors.
When alive, the ponted end of the shell is down in the sand and the "fan" end extends into the water above the sand, making barefoot walking hazardous around a bed of these shells.
The shell is thin when the clam is alive and the shell is closed by flexing, unlike other bivalves.
Two small creatures live inside the shell - a crab ad a shrimp. these feed off the excess that the pen shell pumps inside.