Clean and pick over oysters, using one-third cup cold water; reserve liquor, and add oysters slightly chopped. Clean and pick over clams, reserve liquor, and add to hard part of clams, finely chopped; put aside soft part of clams. Heat slowly to boiling point clams and oysters with liquor from both, strain through cheese cloth. Scald milk with onion, mace, parsley, and bay leaf; remove seasonings, and add milk to stock. Thicken with butter and flour cooked together, add soft part of clams, and cook two minutes. Season with salt and pepper.