Final answer:
A saddle joint is the type of synovial joint that consists of two bones at the base of the thumb, one with a surface that is concave and the other convex.
Step-by-step explanation:
The type of synovial joint that consists of two bones at the base of the thumb, one with a surface that is concave and the other convex, is a saddle joint. At a saddle joint, both of the articulating surfaces for the bones have a saddle shape, which is concave in one direction and convex in the other. This joint allows the thumb to move within the same plane as the palm of the hand or jut out anteriorly, perpendicular to the palm.