• (n.) Any one of the bony plates which support the gill covers of fishes; an opercular bone. • (n.) An operculum.
Refers to the largest bone in the operculum.
O'per·cle noun [ Confer F. opercule . See Operculum .] 1. (Anat.) Any one of the bony plates which support the gill covers of fishes; an opercular bone. 2. (Zoology) An operculum. 
the large bone which forms the upper posterior part of the operculum
an operculum, esp. the posterior bone of the operculum of a fish.