The Turtle Shell is an item given to Alice by the Mock Turtle. It offers no protection from underwater enemies and increases Alice's breath intake underwater. It bears a resemblance to Mock Turtle's shell as Alice received the shell from him.
The Turtle Shell was given to Alice by the Mock Turtle once they arrived at Dry Landing. He then proceeded to leave her on her quest after reminding her to be careful and dived back into the water.
The shell has a dark, lush grass green color that is lighter on the outside and a more golden color at the bottom. It is a huge, stereotypical turtle shell that covers Alice's entire back.
The shell is not worn by Alice any time other than underwater travels, where the animation dissolves on and off of her back. It does not change Alice in any way other than her lung capacity, making her an "honorary reptile."