On my fourth trip to Holden Beach, North Carolina, I discovered something new about Mossy Arks...their hinge markings.

I don't know how they are accomplished...whether they just appear while the shell is growing, or whether something causes them. 

They remind me of cuneiform or some other form of ancient writing, like Ogham or Germanic ancient runes.

If I had ever managed to find both halves, I would have realized that the patterns are replicated on the other side of the shell. You can see this on the following site:

https://www.thefossilforum.com/topic/61076-ark-shells-hinge-markings/