In most turtles the bony shell structure is covered with a layer of horny scutes.

The bony shell, made up of a series of connected individual plates, is well supplied with blood vessels. It grows by deposition of a new layer around the edges of each individual bone, and the scutes grow by deposition of a new horny layer on the surface of the bone. One scute can cover several bones of the shell.
In theory, the age of a turtle can be estimated by counting the number of the wrinkled rings formed around the edges of each scute. The growth of shell in many species from climates with cold winters and warm summers proceeds in cycles, with a new wrinkle formed at the end of the season. However, the scutes of many aquatic turtles peels off frequently, and become thin and smooth.
Also, the scutes of many land tortoises become worn down due to mechanical abrasions, and a captive turtle lacks the annual growth rings found on wild individuals.
Consequently, estimating the age of turtles by counting the growth rings of the scutes cannot be considered a reliable method of judging the turtle's real age.

The individual scutes have their own names, and the following picture describes scutes of the
carapace ( left ) and plastron ( right ).

Carapace N - nuchal C - centrals Co - costals M - marginals S - supracaudals

Plastron I - intergular G - gular H - humeral P - pectoral Ab - abdominal F - femoral An - anal

The scutes in three families: Trionychidae, Dermochelyidae and Carettochelyidae have been eliminated and replaced with soft skin.