In New England, a roundish black clam shell with a mahogany margin and a light-colored inner surface lacking a pallial sinus is that of an Ocean Quahog* (Arctica islandica). The black coating -- known as a periostracum -- can flake off of washed ashore shells, resulting in countless unique designs. Ocean Quahogs are typically no more than 5" wide; the clam pictured here is about 3.5". Unlike some clams who can be dug by hand in mudflats, Ocean Quahogs live in deeper, sub-tidal waters. Harvesting is accomplished by dredging (source). Other names for this clam include Black Clam and Mahogany Clam.
In researching these deep water clams, I also discovered a 2013 report which names a 507 year-old Ocean Quahog "the longest-lived non-colonial animal so far reported" (Science Direct). I had no idea certain clams could live so long!
*Note: Ocean Quahogs are distinct from the more frequently eaten Quahogs (Mercenaria mercenaria) who live in shallower waters primarily south of Maine.