These clam shells were abundant on the beach at Assateague Island last August. I am not versed in bivalve taxonomy, but I believe they are the valves of Mercenaria mercenaria. A characteristic of the species is said to be the purple border along the edge of its shell.
The shells of Mercenaria mercenaria are relatively large, thick and heavy. It is fascinating to ponder on the question of why the clam invests so much calcium, energy and time to build such a fortified pair of shells. Is the function of the shells mostly to protect their occupant from predators?
I collected several of these shells primarily for display. They are now sitting in a large glass bowl in my library. Obviously, they are also good for stimulating one's mind into thinking about evolution.