Shackle Joint from the Exoskeleton of a Siluroid Fish
"A joint involving the principle of the shackle. Specifically, in anatomy, a kind of articulation found in the exoskeleton of some fishes, formed by the passing of a bony ring of one part through a perforation of another part, the two being thus movably linked together." —Whitney, 1889
This illustration shows the shackle joint of a large spine with a bony plate of the skin of a siluroid fish.