In which type of joint does one bone have an ovoid convexity sitting within the ellipsoidal cavity of another bone?

The description given in the question refers specifically to a condyloid joint. In this type of joint, one bone has an ovoid convex surface that fits into the elliptical cavity of another bone, allowing for movement in two planes—flexion and extension, as well as abduction and adduction. This unique structure facilitates a greater range of motion compared to hinge joints, which only allow for movement in one plane, and ball-and-socket joints, which provide a wider range of movement but involve a rounded head that fits more deeply into a socket rather than a simply ovoid shape. Condyloid joints include examples like the wrist joint, which allows for both circular motion and bending/straightening. Irregular joints, on the other hand, have surfaces that fit together in a more complex manner without a specific convex-concave relationship, allowing only limited movement. Thus, the characteristics of the condyloid joint fit the description best.