To add to Dave Campbell's exceptions, we can include the Neritidae.  A study
by Gainey and Wise (1980) of Nerita tesselata and Nerita versicolor found
that although the original embryonic and larval shells (protoconch) are
constructed solely of aragonite, shell secreted by the mantel after the
protoconch is a multi-layered affair with an aragonite layer of complex
crossed lamellar crystals on the inside, an middle aragonite layer of
crossed lamellar crystals, and a calcite outer prismatic layer.  The outer
calcite layer is very thin, most of the shell is still aragonite.

And on the dangers of bleach.  Hear! Hear!  I have ruined several shirts
because I was just going to drop one shell into bleach and I knew I could do
it without a splash - ha!  Also, I second Andrew's comments on it not doing
harm to all but a few shells.  I have forgotten murex, cones, and nerotes in
a 50% solution (or higher) of bleach for almost a month with no ill effect.
However, I wouldn't make it a practice.

Tom E.


> >Like most molluscan shells and almost all gastropods (apart only
> from Epitonioidea), Crepidulidae are composed largely of
> aragonite. There will be some calcite layers, but very thin.<
>
> A clarification: Some other gastropods and other mollusks have a
> thin calcite layer.  Most gastropods and bivalves, including
> Crepidula, are entirely aragonite.  Naticids, some limpets, and
> some muricoideans have an outer layer or partial layer of
> calcite.  Eupteriomorph bivalves such as pearl oysters, oysters,
> scallops, anomiids, and limids have an outer layer of calcite and
> sometimes calcite inner layers as well.  Various extinct mollusks
> also had calcite in their shells.  Major muscle attachment areas
> are always aragonite, even when the rest of the shell is calcite.
>
> >Mussels, for interest (Mytilidae) are largely composed of calcite.<
>
> Mytilus itself is largely calcite, but with some aragonite as
> well.  Other genera in Mytilidae range from entirely aragonite to
> largely calcite.
>
> Calcite and aragonite are two different crystal forms of calcium
> carbonate, just as diamond and graphite are two different crystal
> forms of carbon.
>
>     Dr. David Campbell
>     Old Seashells
>     University of Alabama
>     Biodiversity & Systematics
>     Dept. Biological Sciences
>     Box 870345
>     Tuscaloosa, AL  35487 USA
>     [log in to unmask]
>
> That is Uncle Joe, taken in the masonic regalia of a Grand
> Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse,
> Romance at Droitgate Spa
>