One time a friend and I drove to a scallop-shucking plant in Maine, intending
to search their big dumpster for shells.  We got there just as a truck was
hauling the dumpster away, with several tons of shells in it (mostly
Placopecten magellanicus).  So we followed it, figuring they would take the
stuff to the town dump, and perhaps we could look through the load after they
dumped it.  Well, they went right past the dump, and continued driving almost
40 miles inland.  Eventually they pulled in to a large farm, and dumped the
whole load right in the middle of a big field.  The driver said the farmer
was going to plow the shells into the soil.  It was not only a good source of
calcium ("lime") but also a good source of fertilizer, since the shucking
plant only takes the central muscle of the scallop, and leaves the mantle and
viscera in the shell.
Paul M.