When the steam begins to escape and the whistle sounds, uncover and add the clams, cover and cook for another 2 minutes. Add shrimp and cook until just cooked through, another 3 to 5 minutes.
Add the butter, parsley and lemon wedges to the pot. Cook, covered, for another 2 minutes. Lift the steamer basket to drain, then pour the Seafood Boil out onto newspaper. Ladle the liquid into dipping bowl, serve with bread and lemon wedges.