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