The operculum (_op._, Fig. 20d-h) is cartilaginous and lies medial to the lateral edge of the otic capsule between the pars interna plectri and pars media plectri. The anterior end of the operculum (Fig. 20d) lies at a level corresponding to the posterior part of the posterior acoustic foramen. Posteriorly the operculum increases in size, and the pars interna plectri and pars media plectri are reduced (Fig. 20e-f). At a level corresponding to the posterior border of the posterior acoustic foramen the medial portion of the pars interna plectri disappears and leaves a small lateral rod of cartilage surrounded on all but the ventral side by the operculum (Fig. 20f). The operculum expands medially to merge with the main part of the otic capsule (Fig. 20g). The lateral edge of the operculum expands ventrally and then dorsomedially to form a complete tube. Slightly more posteriorly the cartilaginous lateral edge of the otic capsule, lateral to the operculum, dissipates into connective tissue and finally disappears, leaving the posterior end of the operculum as the most distal element of the otic capsule (Fig. 20h). [Illustration: FIG. 20. Transverse sections through otic capsule: _a_) level of anterior ledge of otic capsule; _b_) anterior level of pars interna plectri and pars ascendens plectri; _c_) level of pars media plectri; _d-f_) successive levels of operculum and pars media plectri; _g-h_) posterior levels of operculum. Abbreviations: _corn. prin._, cornu principalis; _cr. par._, crista parotica; _op._, operculum; _p. asc. pl._, pars ascendens plectri; _p. ext. pl._, pars externa plectri; _p. int. pl._, pars interna plectri; _p. med. pl._, pars media plectri; _sq._, squamosal; _tymp. r._, tympanic ring; _vl. l. ot. c._, ventrolateral ledge of otic capsule.]