Cotylorhynchus, a genus of late Permian synapsids. It's herbivorous and it's huge gut was possibly necessary to digest the nutrient-poor plant life it fed on.
You know what, I feel like the shell part of the turtle is best represented by the "turt" part of the world turtle. It just looks the way "turt" sounds.
So a turtle without it's shell I think should start to be called a "le" (pronounced like in the world turtle), which represents very well, as a sound, a "turt"-less turtle