Alright, John;
   You have made a very persuasive argument for shells not being re-
absorbed in marine environments. So the question remains: where to
dead shells go? The only thing I can think of is that they pile up on
eachother year after year until the weight of them, the time, and the
molecular replacement turns them into  a new layer of limestone. Such
limestone from Ordivician (and surrounding periods of time) here in
Ohio and Kentucky are hundreds of feet thick. Then when the level of
the limestone is raised by the action of plate tectonics, the
limestone is weathered, goes down the rivers, and rejoins the oceans
or makes new deltas.
     To watch this process experimentally probably takes more time
than I have alloted---but I'm willing to try.
            Art