1. (Zo["o]l.) A bivalve mollusk of many kinds, especially those that are edible; as, the long clam (Mya arenaria), the quahog or round clam (Venus mercenaria), the sea clam or hen clam (Spisula solidissima), and other species of the United States. The name is said to have been given originally to the Tridacna gigas, a huge East Indian bivalve.
You shall scarce find any bay or shallow shore, or cove of sand, where you may not take many clampes, or lobsters, or both, at your pleasure. --Capt. John Smith (1616).
Clams, or clamps, is a shellfish not much unlike a coclke; it lieth under the sand. --Wood (1634).
You sure it isn't a regionalism?
Well, I had never heard nor seen that usage in my area before (western New York State) and I've never seen nor heard it since...but I bow to Webster.