What's this doing here...

This is a hollowed out scallop shell I found by the river today...I was puzzled at finding this in a freshwater river so far away from the ocean, I usually see these on the beach. I was intrigued by the way the sand had worn away the surface and showed all the layers in the shell.

Unfortunately, there is no longer a macro photo category, and this didn't fit into any categories...does anyone else think the Photo gallery needs help??

At any rate, enjoy...