Whenever I find a new fossil, I usually put effort into identifying it. Usually, I can turn off my computer with a label for the fossil, and I'll go to bed happy with the new item on my display shelf.
However, these guys have always been at the back of my mind for years, and it really nags at me when I have to explain what they are to someone else. Not anymore, I guess!
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These were found in the Liberty Formation of the well-known St. Leon roadcut in Indiana.
I am thinking they are some kind of internal clam mold, but consider it very unlikely due to an inwards impression that is identical in both fossils.