Hi Steve,
 
That's right - a horny operculum is one made of conchin, the brown, semi-
flexible stuff that most opercs are made of (as in Muricidae, Fasciolariidae,
Littorinidae, Melongenidae, Buccinidae, Strombidae, etc.).  The term
distinguishes this type of operculum from the "shelly" or calcareous type (as
found in Neritidae, Turbinidae, Phasianellidae, and some Naticidae).
 
Paul M.