A tortoise's bony shell is covered with a layer of scutes (plates, shields, scales). These scutes do not correspond in size or shape to the bones below them. Scute attributes can be used to identify tortoises.
Carapace scutes (top) -- A typical tortoise (land turtle) has 5 vertebrals, 4 right and left costals, 11 right and left marginals, 1 nuchal scute, and 1 supracaudal scute.
Plastron scutes (belly) -- A typical tortoise has a pair of gulars, a pair of humerals, a pair of pectorals, a pair of abdominals, a pair of femorals, and a pair of anal scutes.
Sexing -- In many tortoise species, the male has a longer tail, a concave plastron, curved-in supracaudal scute, and sometimes a more angular anal notch (angle between the anal scutes). Each species has its own gender characteristics though.
I used the shell of an angulate tortoise as my model for these illustrations. Just like a typical tortoise, the angulate tortoise carapace has 5 vertebrals, 4 pairs of costals, 10-12 right and left marginals, 1 small nuchal, and a single undivided supracaudal scute.
However, the plastron of the angulate tortoise has a unique feature: a single, protruding gular scute (throat shield). This scute gives the species its popular name bowsprit tortoise. Bowsprit is the long pole in the front of a sailing ship. The angulate tortoise also has a pair of humerals, a pair of pectorals, a pair of abdominals, a pair of femorals, and a pair of anal scutes.
Angulate / bowsprit tortoise carapace scutes (top).
Angulate / bowsprit tortoise plastron scutes (bottom).