If anyone has pondered a shell with an operculum which just CAN NOT
fit into the aperture, the answer is this.

The dealer probably has not supplied the wrong operc. Most gastropods
have opercs which fit reasonably well in the aperture; in some
species it more-or-less blocks most of the aperture, possibly leaving
the canal unsealed. In others (Turbinidae) it hermetically seals the
shell. In some it is far too small to seal.

Well, in some Calliostoma (eg NZ's C. selectum, C. waikanae and other
members of this superspecies) and Entemnotrochus the operc, if dried
flat, is far too big to fit. For instance, in the 194mm
Entemnotrochus rumphii I just bankrupted myself on, the aperture is
56mm high... the operc is about 66mm high. The operc was supplied
out-of-shell, dried loose, so that it curled somewhat. No way would
it fit, even warped.

These opercs act as a diaphragm. When living the operc is saturated
with water and is soft and flexible. When the animal retracts it
drags the operc right in with it, and the operc bends, thoroughly
sealing the shell. I have observed this with plenty of the
Calliostoma named above.

The trick with these is to boil the operc until it is nice and
pliable, fill the aperture with cotton, then align the operc (with
leading growth edge at the outer top of the aperture) and push it in
so that it sinks in evenly. Then allow to dry in place. Worked fairly
well with the big pleurotomariid, though the edges have curled a bit
and it's now a bit loose.

Those pleurotomariids with seemingly-too-small opercs, as well as
turritellids, have evolved a tactic to avoid crab-peeling predation.
These animals retract far up into the shell, past the point where any
crab can break back the shell wall. The operc probably seals fine at
this point.
--
Andrew Grebneff
Dunedin, New Zealand
64 (3) 473-8863
<[log in to unmask]>
Fossil preparator
Seashell, Macintosh & VW/Toyota van nut
________________________________
I want your sinistral gastropods!
________________________________
Opinions in this e-mail are my own, not those of my institution
_______________________________________________
A: Because it reverses the logical flow of conversation.
Q: Why is top posting frowned upon?