Three parts: Carapace: the upper part of the shell Plastron: the lower part of the shell Bridge: a line of bone in between the fore and hind limbs that connects the plastron to the carapace.
Carapace In order for the carapace to form, it needs to drape itself over some kind of supporting structure. Is the fusion of about 50 bones – the ribs and the vertebrae. The dorsal and sacral (vertebrae that make up the middle of the backbone) vertebrae have been fused together and join with the neural bones of the shell.
The scapula (shoulder blade) has joined to the carapace at its outer surface where it can anchor the legs more effectively.
Plastron Is the fusion of bones including the clavicle (collar bones), bones between the clavicles, and portion of the ribs The clavicle (collar bone) has become a part of the plastron known as the epiplastron.
Two layers of the Shell: Plates:• Bony• part of the skeleton• make up the general shape of the shell• also known as the dermal armor
The shell is covered with a layer of keratin, and is arranged in patchescalled, Scutes:• top (outer) layer that covers the shell• formed from skin tissue• gives color and texture• the carapace usually has 38 scutes, and the plastron, twelve to fourteen.• The scutes, however, do not precisely overlap the bones.• Instead, they are staggered, which helps give the shell more rigidity.• These layers are common on six of the seven marine turtles but the leatherback turtle has tough skin, and a reduced number of bony plates, rather than scutes.
Scute patterns: Different species of turtles have scutes of different patterns and designs, and there is often individual differences among members of the same species.
In some, the shell has evolved a cartilaginous hinge in between certain bones.
Mud Turtles: the hinge in between the pectoral and abdominal scutes which allows the turtle to close up the front half of its shell. Hingebacks: the hinge has evolved on the carapace instead of the plastron and lies in between the second and third costals. It allows the tortoises to close the hind quarters of their shells.
Box Turtles: They developed two hinges. One inbetween the pectorals and the abdominals and another inbetween the abdominals and the femorals. Thus allowing the turtles to close up both halves of their shells making an impenetrable fortress or Box
There are also species of chelonian who have decided to mess with their protective covering and make it more maneuvarable and less protective. The bony casing has been reduced so as to allow the animal to move more freely. In some, not only has the bony layer been radically reduced, but the horny keratinous covering of scutes has been dumped in favor of a tough leathery skin. The plastron has been modified into a set of strut like supports instead of a solid casing.
In another species, instead of bony plates there are many small interwoven bones that provide its support. The shell is completely gone. Among other changes in chelonian shell structure there is plastron reductions. These reductions can be nearly to the point of nonexistence and are probably made to allow the turtle to move with greater ease as its chases down frogs and insects.
Shell shapes: The shell shapes of turtles differ with each species, and are often related to habitat. Most aquatic turtles are generally flatter, allowing them to move faster through the water. Tortoises, on the other hand, have carapaces that are dome-shaped.
Shell growth: As a shell grows, the number of scutes generally does not change, but their size does. In some turtles, old scutes are shed and replaced by larger, new ones. In other species, including box turtles, tortoises, and wood turtles, scutes enlarge in diameter as new keratin is laid down.
The "growth rings" in scutes have been used be some experts to help determine the age of a turtle. Age estimation based on growth layers, however, can be erroneous for several reasons: Some turtles produce multiple growth zones per year.
Growth is determined by changes in the environment (seasons), so age determination by examination of growth rings would be more accurate in wild turtles, than those kept in environments which do not change significantly. Growth layers may wear with age, so older turtles may be estimated to be younger than they really are.