The bones of the shell are covered with horny scutes. The division between adjacent scutes is called the seam. A seam often leaves an impression, termed a sulcus, on the underlying bones. Along the anterior midline of the carapace is a single cervical scute. This is followed posteriorly by a series of five vertebral scutes. Along each side and touching the vertebrals is a series of four pleurals. Outside the pleurals and extending along each side from the cervical are 12 marginal scutes (11 in Kinosternidae). In the alligator snapping turtle (Macroclemys) a series of small scutes, called supramarginals, appears between the posterior pleurals and the marginals.

left: Geoemyda spengleri - common black-breasted leaf turtle
(photo: Ron de Bruin);
right: Phrynops geoffroanus geoffroanus - Geoffroy's side-necked turtle
(photo: Carl H. Ernst)

The scutes of the plastron are divided into pairs by a median longitudinal seam. Anteriorly there is a pair of gular scutes (except that in the family Kinosternidae a single gular is usual); in some families an intergular is also present. Paired humerals, pectorals, abdominals, femorals, and anals follow, respectively, and in the Cheloniidae an interanal is sometimes present. At the posterior edge of each axillary notch there may be an axillary scute, and at the front edge of each inguinal notch there may be an inguinal scute. Inframarginals, a series of small scutes lying between the carapacial marginals and the sides of the adjacent plastral scutes, are present in the families Cheloniidae, Chelydridae, Dermatemydidae, and Platysternidae.

Turtles of the genera Pelusios, Emys, Emydoidea, Terrapene, Cuora, Cyclemys, Pyxidea, Notochelys, Pyxis, and Testudo have a transverse hinge, more or less developed on the plastron, and in most species of Kinosternon a pair of hinges borders the abdominals. These hinges allow the plastron to be folded up to enclose the head and limbs if it is large enough.

Pelusios sinuatus - E African serrated mud turtle
(photo: Victor Loehr)

Turtles of the families Carettochelyidae, Dermochelyidae, and Trionychidae have lost the horny covering of scutes, and the bony material in their shells is much reduced. Instead, they have a tough, leathery skin. These turtles are often referred to as leatherbacks or softshells.

Another exceptional characteristic of turtles is the migration of the limb girdles to positions inside the rib cage (which, along with the vertebral column, helps form the shell). The limbs of turtles are adapted to the medium through or on which they travel. For instance, the terrestrial tortoises have evolved elephantine hind limbs, which help to support them, and shovellike forelimbs, which aid in digging. The semiaquatic turtles have developed various degrees of webbing between the toes; generally, the more webbing, the more aquatic the turtle. The limbs of the marine species and Carettochelys are modified as flippers with which they quite literally fly through the water (see the movie of Chelonia mydas, the green turtle).

Chelonia mydas agassizii - Pacific green turtle
off the coast of Borneo
(film: Magnus Stengard)