Wash clams under cold running water until free of sand. Scrub with abrasive sponge or cloth to remove remaining grime.
Boil the water in a large pot and add clams. Cover the pot and lower the heat to minimum. Steam until clams barely open. Stir occasionally so that clams get cooked evenly.
Remove clams from the pot, reserving broth. Strain the broth through cheesecloth. Place one-half cup broth in a small bowl for each serving. About two cups of broth will be left. Reserve to serve separately.
Melt butter and add lemon juice, salt and pepper to taste, and a little white wine if desired. Divide among small bowls and serve with a slice of lemon or a sprig of watercress.
Heap clams into soup bowls and serve with dry white wine. Provide fingerbowls and a large bowl for shells.