cleithrum noun1 •A 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 formscleithra
