The plastron is the nearly flat part of the shell structure of a tortoise, what we would call the belly, similar in composition to the carapace; with an external layer of horny material divided into plates called scutes and an underlying layer of interlocking bones. In certain families there is a hinge between the pectoral and abdominal scutes allowing the turtle to almost completely enclose itself. In certain species the sex of a testudine can be told by whether the plastron is concave, male; or convex, female. This is because of the mating position.
The scutes
See also
"Plastron" is also the name of a half-jacket used in the sport of Fencing.