Cheloniid turtles swim by moving the blade of their pectoral flipper up and down along a line inclined from 40? to 70? from the horizontal plane. The leading edge of the blade is inclined anteroventrally on the downstroke and anterodorsally on the upstroke, and the tip of the blade usually inscribes a figure 8. The downstroke is the main propulsive stroke, but some propulsive components are generated on the upstroke. The blade, composed of the antebrachium and manus, is driven largely by movements of the humerus that include on the downstroke a retraction of 70?-800, a 60?-700 rotation, and a 30?-50? adduction. A shift in the axis of the articular surface of the humeral head, relative to the axis of the shaft, enables a retraction of the humerus to bring about concurrently most of the adduction and much of the rotation observed. The crus and