<P> Overlying the boney elements are a series of scutes, which are made of keratin and are a lot like horn or nail tissue . In the center of the carapace are 5 vertebral scutes and out from these are 4 pairs of costal scutes . Around the edge of the shell are 12 pairs of marginal scutes . All these scutes are aligned so that for the most part the sutures between the bones are in the middle of the scutes above . At the anterior of the shell there may be a cervical scute (sometimes incorrectly called a nuchal scute) however the presence or absence of this scute is highly variable, even within species . </P> <P> On the plastron there are two gular scutes at the front, followed by a pair of pectorals, then abdominals, femorals and lastly anals . A particular variation is the Pleurodiran turtles have an intergular scute between the gulars at the front, giving them a total of 13 plastral scutes . Compared to the 12 in all Cryptodiran turtles . </P> <P> The carapace is the dorsal (back), convex part of the shell structure of a turtle, consisting of the animal's ossified ribs fused with the dermal bone . The spine and expanded ribs are fused through ossification to dermal plates beneath the skin to form a hard shell . Exterior to the skin the shell is covered by scutes, which are horny plates made of keratin that protect the shell from scrapes and bruises . A keel, a ridge that runs from front to the back of the animal is present in some species, these may be single, paired or even three rows of them . In most turtles the shell is relatively uniform in structure, species variation in general shape and color being the main differences . However the soft shell turtles, pig - nose turtles and the leatherback sea turtle have lost the scutes and reduced the ossification of the shell . This leaves the shell covered only by skin . These are all highly aquatic forms . </P> <P> The evolution of the turtle's shell is unique because of how the carapace represents transformed vertebrae and ribs . While other tetrapods have their scapula, or shoulder blades, found outside of the ribcage, the scapula for turtles is found inside the ribcage . The shells of other tetrapods, such as armadillos, are not linked directly to the vertebral column or rib cage allowing the ribs to move freely with the surrounding intercostal muscle . However, analysis of the transitional fossil, Eunotosaurus africanus shows that early ancestors of turtles lost that intercostal muscle usually found between the ribs . </P>

The dorsal part of a turtle's shell is called the