When you are digging in the right area, you will find whole beds of clams like these two. The neat thing is that all of the clams with calcite in them will be oriented the same way --- with the hinge near the top. This is a natural position for a living clam.

If you find a clam lying on its side or oriented somehow randomly, it will almost always be hollow (meaning there are no calcites inside). What this says to me is that the clams were buried suddenly while still alive. For some reason, only the living clams formed calcite crystals --- empty shells buried at the same time never developed calcite. To me it suggests that the body of the animal actually had some effect on the formation of the calcite. I would love to hear from a geologist or someone who knows more about these things than I do to comment on this...