Most North American hardshell clams, or quahogs, belong to a single species, Mercenaria mercenaria, but the different names they go by can be confusing. At one time these monikers reflected the clams' place of origin, like Little Neck on Long Island or Cherrystone Creek in Virginia. But nowadays their names, unlike those of oysters, denote only size.
Chowder clams are the largest of the hardshell clams. (Confusingly, chowders are also called quahogs, a name often used for referring to hardshell clams in general.) These big bivalves exceed three inches in diameter. Cherrystones **are the next smallest; they measure from two and a half to three inches. Even smaller, **littlenecks are one and a half to two inches wide.
As a general rule, the smaller the clam, the more delicate and tender the meat, which is why littlenecks are so often eaten raw. (Two other species of hardshell clam, not shown, are seldom available for retail sale: Spisula solidissma, or surf clams, and Artica islandica, or ocean quahogs. The former are often used in chowders, and the latter are typically used for fried clam strips.)
Mya arenaria, the softshell clams are usually called steamers, softshell clams have an especially prominent necklike siphon that protrudes from their delicate shells. Delicious when steamed and dipped in drawn butter, they're also the most common choice for fried clams.
When buying hardshell clams, choose ones with tightly shut shells that haven't turned white (a sign of age). Knock two together: if they sound like steel balls clacking, they're fresh; if hollow sounding, they're old. Softshell clams should be plump, and their siphons should be snugly tucked inside their shells. Hardshell clams can be kept for four to five days in the refrigerator, in a colander set in a bowl and covered with a damp towel. Softshell clams are best enjoyed within 24 hours of purchase.