For soft-shell clams: pisser clams or steamers.
Briny, succulent, and sweet, clams are bivlave mollusks from the sea. They're delicious raw, grilled, steamed, and stuffed and baked, as well as in clam chowder and pasta.
Clams are separated into two categories—soft-shell and hard-shell. Hard-shell clams come in many shapes and sizes. On the Atlantic coast where clams reign, the most common variety of hard-shell clam is the quahog (pronounced KWAH-hahg) with its thick, tough, pale-colored shell. Quahogs are sold according to size: chowder clams being the largest, then cherrystones, followed by littlenecks and countnecks. Mahogany clams are another variety of hard-shell clam, easily recognizable by the reddish-brown color of their shells. Soft-shell clams have a shell that's thin and brittle. These clams have a dark neck (or foot, as it's sometimes called) that protrudes from the shell and keeps them from closing tightly. Soft-shell clams are never eaten raw; the most common way to prepare them is by steaming or frying Razor clams, less often seen at the market, get their name form their unique shape; they look like old-fashioned straight razors.
At the fish counter, use your eyes and your nose to guide you. Fresh hardshell clams should look tightly closed or just slightly gaping open. Make sure their shells are closed or that they close immediately with a gentle tap. That's an indication that they're still alive. If they're yawning wide, they're dead, or nearly so. Once you have them in hand, take a sniff. They should smell like the sea. If they're really fishy smelling, don't buy them.
Discard any whose shells open prior to cooking.
Buy more than the quantity required, since you’ll likely have to discard a few that don’t open during cooking.
Just before cooking hard-shell clams, look for any that have opened and tap them on the counter. If they don't close, discard them. Once you've weeded out the bad ones, scrub the remaining clams under cold running water with a stiff brush to get rid of any grit. Soft-shell clams also tend to collect more sand and grit than other clams, and many recipes will instruct you to first soak them in a bowl of cold salted water for a few hours to purge the sand. When steaming, soft-shelled clams, most cooks skip the soaking step and simply serve the steamed clams with a bowl of clam broth (the liquid they were cooked in) for dipping to rinse off any grit.
Keeping them fresh: Store in an open plastic bag (shellfish will suffocate in a sealed bag) in the refrigerator on a bed of ice in a large bowl or dish with sides. Refresh ice as it melts. It's best to cook them as soon as possible, but if they were fresh to begin with, they should keep stored this way for up to two days. Because soft-shell clams gape open, they're highly perishable and should be cooked within a day of purchase.