<In families where the operculum is present, is it present in all
species? For example, do all trochids and turbinids have opercula?>
 
Yes, all trochids and turbinids have an operculum.  Most marine gastropod
families do, but a number of families do not.  Families lacking an operculum
include Bullidae, Cancellariidae, Trichotropidae, Cypraeidae, Triviidae,
Ovulidae, Marginellidae, Harpidae, Haliotidae, and the various "limpet"
families, including Acmaeidae, Patellidae, Fissurellidae, Calyptraeidae and
Siphonariidae.  A few gastropod families, including Volutidae, Olividae, and
Turridae, include both operculate and non-operculate species.  In Janthinidae,
the purple sea snails, there is an operculum in the embryonic snail, but not
in the mature form.
Paul M.
Rhode Island, U.S.A.