Hard shell clams are common on the Atlantic coast and are the clams with blue-grey shells and chewy meat, often used in chowders in the US. They are of the same species as the Venus clams and include cherrystone clams, chowder clams, littleneck clams and topneck clams. It was useful to find this breakdown on Wikipedia: “The smallest legally harvestable clams are called countnecks, next size up are littlenecks, then topnecks. Above that are the cherrystones, and the largest are called quahogs or chowder clams”.