There are a number of varieties of clam.  All are round and stone-like except the Razor clam.  Amande, Hardshell, Venus and
Razor Clam are the most common varieties.

Palourdes (or Carpet Shell) are considered the finest Clam and command a much higher price. Once washed and scrubbed, they are used in some classic dishes such as Linguine alle Vongole and New England Clam chowder. 

They can also be served raw like Oyster, provided they are harvested from Class A waters or have been purified prior to sale.