Turtle shell

Shell
Main article: Turtle shell
Photograph of one half of a turtle skeleton, cut in half vertically
Cross-section of a tortoise skeleton
The shell of a turtle is unique among vertebrates and serves to protect the animal and provide shelter from the elements.[18][19] It is primarily made of bone, and consists of two parts, the carapace which usually contains 50–60 bones and covers the back of the animal while the plastron has 7–11 bones and covers the belly. They are connected by lateral extensions of the plastron. The carapace is fused with the vertebrate and ribs while the plastron is formed from cerain bones of the shoulder girdle, sternum, and gastralium.[18] The shoulder girdle was long thought to lie inside the rib cage, a unique feature, but it actually lies in front of the rib cage; the shoulder blade is oriented vertically, as it was primitively in other reptiles.[20]

The outer surface of the shell is covered in epidermal scales known as scutes which are made of keratin, the same substance that makes up human hair and fingernails. Typically, a turtle has 38 scutes on the carapace and 16 on the plastron; 54 in total. Carapace scutes are divided into “marginals” around the margin, “vertebrals” over the vertebral column (the first one by the neck is called the “cervical”), and “pleurals” between the marginals and vertebrals. Plastron scutes include gulars (throat), humerals and abdominals. Turtle scutes usually interlock like mosaic tiles, though in some species, like the hawksbill sea turtle, the scutes on the carapace can overlap.[21]