Plastron

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).

Indicating that the abdominal and gular scute seams are approximately the same length and the femoral and pectoral seams are approximately the same length. Comparison of the plastral formulas provides distinction between the two species.

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

Other Meanings

Plastron is a name that has also been given to various coverings or structures on the ventral (belly) side of some creatures, people or objects.

"Plastron" is the name of a broad piece of material placed on the chest of certain military uniforms and held in place by two rows of buttons. The plastron was a popular feature of military dress during the period 1800-1914, when it was commonly worn by lancer and uhlan regiments in various European armies. It was also worn by light dragoon regiments in the British Army and by infantry and artillery units in the Imperial Russian Army. Plastrons were usually in contrasting colours to those of the tunics or coats that they were attached to.
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. On many aircraft, this bulge also helps house the main undercarriage when retracted.
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.
"Plastron" is also the name of a one-arm, half-jacket used in the sport of fencing.
It is also the name of a chest guard, used in the archery, preventing the bowstring from being obstructed by the archer's body or clothing as it is released.

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