Art Weil wrote,
>  there are molluscs (gastropods, 'monoplacophorans',  bivalves and
>smaller groups) in the Cincinatian limestones, but because they had
>aragonitic shells, the preservation is very poor compared to the
>calcitic brachiopods, corals, echinoderms and bryozoa.

And Andrew Grebneff replied,
> Actually, I believe corals (true modern hexacorals) are aragonitic. Like
the majority of molluscs, they are diagenetically removed (dissolved) in our
regional Oligocene greensand/limestone formation, though occasionally
phosphatic ghosts of the interseptal spaces are present.

Paleozoic corals include two main groups, the Rugosa and Tabulata, both
originally calcitic. Rugosans can be solitary ('horn corals') or colonial;
tabulates are all colonial. Like modern hexacorals, rugosans have vertical
internal partitions, but they have a basically 4-sided symmetry instead a
6-sided one. The tabulates have horizontal internal partitions. Unless ideas
have changed about the evolution of corals, neither one is the ancestor of
the modern scleractinians (hexacorals), which can be solitary or colonial
and have aragonitic skeletons with vertical internal partitions. That
end-Permian extinction was a doozy.

Cheers and best holiday wishes, with flying pigs for all!
Andy

Andrew K. Rindsberg
Geological Survey of Alabama