What is "buckler thorn"

Buckler \Buc"kler\, n. [OE. bocler, OF. bocler, F. bouclier, a shield with a boss, from OF. bocle, boucle, boss. See Buckle, n.]

1. A kind of shield, of various shapes and sizes, worn on one of the arms (usually the left) for protecting the front of the body.

   Note: In the sword and buckler play of the Middle Ages in England, the buckler was a small shield, used, not to cover the body, but to stop or parry blows.

2. (Zo["o]l.)

   1. One of the large, bony, external plates found on many ganoid fishes.

   2. The anterior segment of the shell of trilobites.

3. (Naut.) A block of wood or plate of iron made to fit a hawse hole, or the circular opening in a half-port, to prevent water from entering when the vessel pitches.

   Blind buckler (Naut.), a solid buckler.

   Buckler mustard (Bot.), a genus of plants ( Biscutella) with small bright yellow flowers. The seed vessel on bursting resembles two bucklers or shields.

   Buckler thorn, a plant with seed vessels shaped like a buckler. See Christ's thorn.

   Riding buckler (Naut.), a buckler with a hole for the passage of a cable.