Hi Wes,

An excellent paper covering many aspects of opercular formation is:

Constructional morphology, origin, and evolution of the gastropod operculum

By: Antonio G. Checa and Antonio P. Jimenez-Jimenez

In: Paleobiology, 24(1), 1998, pp. 109-132

In spite of the journal title, the paper doesn't particularly deal with
fossil forms.  It has a lot of general information applicable to both
fossil and recent forms.  I do have one extra xerox copy, which I can
send you if you would like it.

Regards,
Paul