Interpreting clam shell structure...
This is a polished Mercenaria mercenaria shell. Mercenaria is the cherry stone hard shell clam. It's a clam that you generally get from the east coast in cans. Unfortunately, there have been some reports that Mercenaria might now be on the west coast. That's not good. It shouldn't be here. But this is a polished specimen of Mercenaria and what we'll notice, I think the most obvious feature on this specimen--we purchased this at the Tucson gem and mineral show last year--are these very obvious bands of varying widths and color intensities all along the surface. Now, the way that this animal grows is from this direction. The beak of the shell or the end toward the edge. So this represents actually the larval shell of the animal which then, when we collect the shell, is the oldest part of the shell. This is the bit of shell that was deposited right before death. The shell grows by adding an increment of calcium carbonate onto the edge. So we refer to this as incremental growth. This is very important.
If we took a section along the growth transect, the direction of growth of the shell, this is what it looks like. This is actually a very thin slice of one of these shells. This is actually a shell from the delta of California. It's been thin sectioned, polished very well and then photomicrographed. Here's a growing edge of the shell, so as we move back in this direction, we're actually moving back toward the beak or the larval shell. And what you'll notice are these bands running through the shell. You can see some of these bands up here. Each one of these bands represents, depending on where in the world and what type of environment this shell came from, represents on average a single day of growth or half a day's growth, depending on whether the tides are daily or diurnal tides.