The main physical characteristic of a turtle is its shell, but turtles also have some other, less obvious unique features, such as beaks and a skull that lacks open spaces where other reptiles possess them. The shell consists of highly modified ribs, vertebrae and other bones.
A turtle's shell is definitely its most noticeable unique feature. It is not a single piece, nor can it be shed like the shell of a lobster or crab. The shell consists of many fused bones that combine into two larger pieces of armor: one on the animal's back and another on the animal's belly. These bones include flattened ribs and clavicle bones. A turtle's shoulder girdle and pelvic girdle are entirely within this bony shell.
Turtles also have odd skulls, as compared to their reptiles the snakes and lizards. They lack skull openings called fenestrae, giving them a helmet-like appearance, and instead of teeth, they have a keratinized edge along their jaws, forming a beak. While no living species has teeth, some have evolved adaptations for chewing or gripping food. For example, sea turtles have evolved esophageal papillae -- spikes of cartilage that line their throat. They use these to grip slippery jellyfish.