Fig. 4. Pelvic fin motion. Paired pelvic fins oscillated in a regular contralateral cycle. (A–D) Four stages of left pelvic fin oscillation are drawn in lateral, transverse and ventral views. The left fin is coloured green, the dorsal side of the fin in light green and the ventral side of the fin in dark green. (A) Column A is arbitrarily assigned as the start of the oscillating cycle when the fin is adducted against the body relative to the transverse plane (phase=0 deg.). (B) As the fin started its cycle the fin's lateral edge began abducting toward the transverse plane and supinated away from the sagittal plane where it reached maximum lateral excursion (phase=120 deg.). (C) As the fin continued abducting toward the transverse plane, the fin's lateral edge started pronating medially toward the fish's mid-sagittal plane (phase=180 deg.). (D) The fin began adducting away from the transverse plane as the fin's lateral edge was pronated maximally toward the mid-sagittal plane (phase=300 deg.). Finally, the fin made its return stroke back to a fully adducted, partially supinated starting position (A, phase=360 deg./0 deg.). Arrows in each of the images note the direction in which the fin is moving or about to move. Background colours of each panel correspond with those in Fig. 3.