It is replaced by the top card on the deck or the previous card on the discard pile.
Play continues by discarding one card at a time until a player knocks twice on the table.
After the knock the players get one last discard. The hands are then shown and the hand closest to 31 wins. A player who hits 31 exactly wins automatically and does not have to wait for the knock or make a knock. I wonder whence this knocking aspect is derived. Again, I find this interesting, but I am not certain of its period accuracy.