>A clarification: Some other gastropods and other mollusks have a
>thin calcite layer.  Most gastropods and bivalves, including
>Crepidula, are entirely aragonite.  Neritids, some limpets, and some
>muricoideans have an outer layer or partial layer of calcite.

All of these shells disappear entirely in sediments where aragonitic
shells are dissolved, and leave no traces of their existence other
than (rarely) phosphatic internal casts (steinkerns). Neritids don't
normally occur in our fossil record, except for one locality; however
I have colected fossils of the family in Fiji which were entirely
decalcified, leaving molds... which preserved the color patterns!

>  Eupteriomorph bivalves such as pearl oysters, oysters, scallops,
>anomiids, and limids have an outer layer of calcite and sometimes
>calcite inner layers as well.

These taxa remain is such sediments, complete with muscle scars and
inner layers. Propeamussiids too.

>Mytilus itself is largely calcite, but with some aragonite as well.
>Other genera in Mytilidae range from entirely aragonite to largely
>calcite.

The only mytilids we find in our late Oligocene limestones (in which
in most localities the aragonitic shells are gone) are an undescribed
large Mytella, though Modiolus does occur in the few localities where
aragonitic shells are preserved.
--
Andrew Grebneff
165 Evans St, Dunedin 9001, New Zealand
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