saddle-joint

from The Century Dictionary.

• noun A joint made by turning up the edges of adjacent plates of tin or sheet-iron at right angles with the bodies of the sheet (one margin so turned up being nearly twice as wide as the other), and then turning down the broader margin snugly over the other so that the margins interlock.
• noun In anatomy, a joint where the articular surfaces are inversely convex in one direction and concave in the other, admitting movement in every direction except axial rotation.