Dictionary of Ichthyology  

preopercle = the L-shaped dermal bone which lies in front of the gill cover and which bears the upper part of the preoperculo-mandibular lateral line canal. It may bear spines or fine teeth. It belongs to the suspensorium rather than the opercular series and acts to prevent the suspensorium from moving laterally. Also called preoperculum.

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