Four club hooks were examined with the scanning electron microscope. Their relative sizes are seen in the photograph below. The ninth hook in the ventral series, counting from the proximal end of the club (abreviated as hook V9) (far right in photograph) was the largest club hook. Its mate from the dorsal hook series (hook D9) is seen on the far left. Between these two are hooks V12 (left of center, smallest of the ventral series hooks examined) and V10 (right of center). Note the near bilateral symmetry of the ventral series hooks.
Figure. Frontal views of hooks of O. robusta, left to right: D9, V12, V10, V9, 100 cm ML, NMNH #335984 (clubs only). Photographs by R. Young.
Figure. Side views of the four hooks, D9 (left bottom), V9 (left top), V12 (right bottom), V10 (right top). Photographs by R. Young.
Figure. Attachment-site views of hooks V9 (left) and V12 (right). Photographs by R. Young.
Figure. Two views of hook D9. Left - top-frontal view. Right - attachment-site view. SEM photographs by R. Young.
Of the three species of Onykia examined with the SEM, the general appearance of O. robusta is most similar to O. robsoni.