The Mock Turtle tells Alice about his school years growing up under the sea, but is interrupted by the Gryphon who ask him to explain a dance called the Lobster Quadrille. Of course it is as nonsensical as you could expect at this point and the Mock Turtle and the Gryphon act out the dance (sans lobsters) much to their own amusement.
The Mock Turtle does this while singing a song about a whiting and a snail, who are both invited to the dance but quite sensibly decline. Alice recites some verses herself after some encouragement and finally the Mock Turtle entertains them by singing a song about soup.
They hear a shout that the trial is beginning, and the Gryphon takes Alice by the hand to attend - leaving the Mock Turtle to sing to himself.
