cleithrum

plural cleithra

1. biologyA large bone in fishes, which extends upward from the base of the pectoral fin and anchors to the cranium above the gills, forming the posterior edge of the gill chamber.

   The secondary arch consists of four pieces of bone, the posttemporal, supracleithrum, cleithrum and postcleithrum.[…]The cleithrum is the largest piece and the other elements are attached to it.

   1993, J. J. Videler, Fish Swimming, Springer, page 66

   2 more examples

   The main components of which are, from dorsal to ventral, the post-temporal, supracleithrum, cleithrum and the postcleithra.[…]The cleithra of either side of the body are connected with the pharyngeal jaw apparatus and with the urohyal by the bilateral pharyngocleithralis musculature and the median sternohyoideus muscle.

   2000, Melanie L. J. Stiassny, “Chapter 6: The Skeletal System”, in Gary Kent Ostrander, editor, The Laboratory Fish, Academic Press, page 116

   In Ichthyostega (Figs 5.2, 5.11), Acanthostega (Fig. 5.20), and also in Hynerpeton (Fig. 5.28C, D) and Ventastega, the cleithrum was relatively reduced, though it was still a substantial dorsally extensive blade.

   2012, Jennifer A. Clack, Gaining Ground: The Origin and Evolution of Tetrapods, 2nd edition, Indiana University Press, page 212