Tortoises as well as turtles are in fact the only reptiles with hard and bony shells. To no surprise, the shell works as a protective armor from the harm of the environment. The outer layer - carapace - is the shell that we see on the animal and beneath it hides the inner bony layer, then followed (looking down) by the rib bones. The lower shell of the tortoise is called plastron. If you ever were wondering why these lovely animals walk so slow - try imagining walking around yourself with a heavy protective armor, as it most certainly weighs the animal down.