Dictionary of Ichthyology  

opercle = the principle and largest, paired dermal bone comprising the upper part of the gill cover above the subopercle. Sometimes used in aging studies. Also called operculum, q.v.

Hyoid and opercular apparatus of Gadus, B = branchiostegals, E = os entoglossum, H = hyoid, HM = hyomandibula, I = interoperculum, O = operculum, P = preoperculum, S = suboperculum, from Kingsley's "Textbook of Vertebrate Zoology, 1906. Photograph by Brian W. Coad.

Canadian Museum of Nature