Dear Nora
inside one genus , if one species have operc. all other have also (beware
of large familys like"volutes".They are many genus , some have operc and
some lack it)
Cypreas don't need perio because when the animal is active
the shell is covered by the mantle (the part of the animal that secrete the
shell material) . Cones do possess one of the strongest perio of all , it
need long bath in clorine to get rid of it . Since it make the pattern
invisible , shell
dealers obviously clean it up carefully . Periostracum
protect the shell from erosion (by scraping against rocks
and sand) and from attaching organisms like briozoans ,
calcareous algae , barnacles etc. , and from drilling
organisms like sponges , bivalves etc. Obviously the perio doesn't last for
ever , this is why the apex (the
oldest part of the shell) is generally devoid of perio and eroded , drilled
, encrusted etc. while the last
worl is, under the encrustation , still with perio , and under it with fine
color and fine condition .
The operculum serves as door generally , but in the larger strombidae (big
conchs , lambis) is used also for locomotion (they point it in the sand and
jump kicking)
and as a weapon against unworried collectors (it is sharp
and can hurt the palm of your hand) . Some genus of shells have little
operc that seems meaningless , I
think they are currently under evolution : the operc is no longer useful to
their living and they will lost it
in some million years of evolution (if we don't stop it).
 .
 
strombus and cardium collector , I would like to excange
shells , and papers about them . Virgilio Liverani
Ravenna , north Italy  [log in to unmask]