I found this object at the beach on Honeymoon Island in Florida last month. It is the egg case of a marine snail, specifically one of the snails known as whelks (family Buccinidae). I can't get any more specific than that, for I don't know much about whelks.
Nor do I know what these cases are made of. They don't seem to contain calcium carbonate, yet they are quite resistant to decay. This particular one apparently sat on the bottom of the sea long enough for what appear to be bryozoan colonies to spread on its surface.
A snail does not produce an egg case like this one as a complete unit. What seems to happen is that the snail makes one unit (capsule) at a time while stringing them up on a cord one by one to end up with this structure.