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The plastron is the nearly flat part of the shell structure of a turtle or tortoise, what one 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 plastral scutes join along a central seam down the middle of the plastron. The relative lengths of the seam segments can be used to help identify a species of turtle or tortoise. There are six laterally symmetric pairs of scutes on the plastron: gular, humeral, pectoral, abdominal, femoral, and anal (going from the head to the tail down the seam).

As an example, the so-called plastral formula^[1] for the eastern box turtle (Terrapene carolina) is:

 an > abd > gul > pect > hum >< fem

where the notation hum <> fem means that the humerol seam is approximately the same length as teh femoral seam.

The plastral formula for the ornate box turtle (Terrapene ornata) is:

 an > abd >< gul > fem >< pect > hum

indicating that the abdominal and gular scute seams are approximately the same length and the the femoral and pectoral seams are approximately the same length. Comparison of the plastral formulas privides for the distinquishing between the two species.

The plastral formula for the common musk turtle on this page is:

 abd > an > pect >< fem >< hum > gul

Turtle plastrons were used by the ancient Chinese in a type of divination called plastromancy. See also oracle bones.

• "Plastron" is also the name of a half-jacket used in the sport of fencing.
• "Plastron" is also the name of the bulge on an aircraft, such as the Airbus A340, under the fuselage between the wings that is used to correct airflow around the engine pylons.
• A "plastron" is also a thin permanent layer of air around the body of some aquatic insects, held in place by hydrophobic hairs, and acting as a physical gill.

1. ^ Ernst, Carl H. and Barbour, Roger W. 1989. Turtles of the World. Smithsonian Institution Press. Washington, DC.

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