The trochlea is the structure on the distal humerus that articulates with the ulna, and allows for flexion and extension at the elbow. The capitulum is the structure on the distal humerus that articulates with the radius. When the radius rotates during pronation and supination movements, the radial head spins on the capitulum. Among primates, hominoids and other suspensory primates have a rounded capitulum that is separated from the trochlea by a ridge of bone (called the lateral trochlear ridge). A rounded capitulum reflects a very mobile radio-ulnar joint that permits a large range of pronation-supination. Quadrupedal primates, however, have a flattened capitulum that is nearly continuous with the trochlea. This capitular morphology permits only limited pronation-supination, helping to stabilize the radio-ulnar joint during walking and running.   Examine the capitulum on the primate humeri in the picture below. Fill in the blanks in the following statements: A has a rounded capitulum and B has a flat capitulum. A has a capitulum that is separated from the trochlea by a lateral trochlear ridge, and B has a capitulum that is nearly continuous with the trochlea. A is most likely Suspensory , and B is most likely Quadrupedal .