Hello Wes,

Actually, the operculum is not a product of the mantle.  It is produced
by a separate organ, the opercular disc, a pad of specialized secretory
epithelial cells located right where the operculum is attached - on the
posterior, dorsal aspect of the foot.  The opercs of some species are
secreted spirally, with new material being deposited on one edge only,
and the operc gradually rotating on the opercular disc as it grows.
Others are secreted concentrically, with new conchiolin being deposited
on all edges of the operculum simultaneously, without rotation of the operc.

Regards,
Paul M.