Humerous (upper arm) Ulna and Radius which turn over each other and are paralell when you are at anatomical position. I can't quite remember, but I'm pretty sure that Radius is the one that connects to the thumb.
Your radius and ulna are in your lower arm, and the humerus is in you upper arm. The radius goes from your thumb to your humerus, crossing over the ulna which goes from your pinky to your humerus....