Turtles are a type of reptile known as a Testudine, and they are distinguished by a unique shell made primarily of their ribs. The upper part of a turtle shell is the domed carapace, while the underside is the flatter plastron or belly-plate. Its outer surface is covered in keratin scales, the same material found in hair, horns, and claws. The carapace bones form from ribs that grow sideways into broad flat plates that join together to cover the body. A turtle's shell is unique among vertebrates and serves to protect the animal as well as provide shelter from the elements. The flexibility of shells varies. Some species, such as box turtles, lack lateral extensions and instead have fully fused or ankylosed carapace bones. Several species' shells have hinges, usually on the plastron, that allow them to expand and contract. Because of bone loss, softshell turtles have rubbery edges. The leatherback turtle's shell is boneless, but it has thick connective tissue and an outer layer of leathery skin. The most specialised for swimming are sea turtles and pig-nosed turtles. Their forelimbs have evolved into flippers, while their hind limbs have evolved into rudders. The front limbs propel the body forward, while the hind limbs act as stabilisers.