Ah Hah! Warning, spoiler...
Actually, "saddle" is correct, but hopefully this will keep the game alive a little longer.
A saddle is not only simultaneously convex and concave, but it is concave in one dimension (fore and aft) and convex in the other (athwartships).
The same can be said for the inner surface of a torus. However, the outer circumference is convex in both directions. Which means at some point in between it is convex in one direction and flat at right angles.
BTW, can the terms convex/concave be rigorously defined?