also been historically known as Chelonii (Latreille 1800) and Chelonia (Ross and Macartney 1802), which are based on the Ancient Greek word for tortoise: (). Testudines is the official order name due to the principle of priority. The term chelonian is used as a formal name for members of the group. Anatomy and physiology Size The largest living species of turtle (and fourth-largest reptile) is the leatherback turtle, which can reach over in length and weigh over . The largest known turtle was Archelon ischyros, a Late Cretaceous sea turtle up to long, wide between the tips of the front flippers, and estimated to have weighed over . The smallest living turtle is Chersobius signatus of South Africa, measuring no more than in length and weighing . Shell The shell of a turtle is unique among vertebrates and serves to protect the animal and provide shelter from the elements. It is primarily made of 50–60 bones and consists of two parts: the domed, dorsal (back) carapace and the flatter, ventral (belly) plastron. They are connected by lateral (side) extensions of the plastron. The carapace is fused with the vertebrae and ribs while the plastron is formed from bones of the shoulder girdle, sternum, and gastralia (abdominal ribs). During development, the ribs grow sideways into a carapacial ridge, unique to turtles, entering the dermis (inner skin) of the back to support the carapace. The development is signaled locally by proteins known as fibroblast growth factors that include FGF10. The shoulder girdle in turtles is made up of two bones, the scapula and the coracoid. Both the shoulder and pelvic girdles of turtles are located within the shell and hence are effectively within the rib cage. The trunk ribs grow over the shoulder girdle during development. The shell is covered in epidermal (outer skin) scales known as scutes that are made of keratin, the same substance that makes up hair and fingernails. Typically, a turtle has 38 scutes on the carapace and 16 on the plastron, giving them 54 in total. Carapace scutes are divided into "marginals" around the margin and "vertebrals" over the vertebral column, though the scute that overlays the neck is called the "cervical". "Pleurals" are present between the marginals and vertebrals. Plastron scutes include gulars (throat), humerals, pectorals, abdominals, and anals. Side-necked turtles additionally have "intergular" scutes between the gulars. Turtle scutes are usually structured like mosaic tiles,