Clams are sold in three forms: In the shell, shucked, and canned (frozen of shelf stable)

Clams In Shell

Usually Sold by the dozen or pound.

They should be alive when purchased.  Test by making sure that open shells close when handled. If they do not close they are dead and no longer usable. 

Shucked clams

The clam meat that has been removed from the shells.  clam meat is usually sold by the pint or quart.  The shucked clams should be plump, with clear liquor, and free from shell pieces.  Freshly shucked clams are packed in waxed cartons and metal containers and should be refrigerated and surrounded by packed ice.  If handled correctly and proper temperatures maintained, clam meat can stay fresh for a week to tem days.

Quick Frozen shucked clams are available but care should be taken to thaw correctly and once thawed should not be refrozen.

Canned Clams

Varieties of clams are canned in whole, minced, or as chowder, and are packed in various sizes of cans from 3 1/2 ounces to 4 pounds.  Clam juice, broth, and nectar are also available canned or bottled.