This is a nice eastern musk turtle (Sternotherus ordoratus) I found last week. He as crossing the road and my keen eyed navigator pointed him out. We turned around and quickly moved him off the road. Eastern musk turtles are primarily aquatic and carnivorous. They are often opportunists and will eat a varied diet of snails, crayfish, little fish (if they can catch them), and other invertebrates. I rarely see them so I was very excited!
With that algae on the shell, it looks quite like a rock! I wish I was better at knowing all my North American turtles. There are quite a few and they can be hard to tell apart unless they are in your hand. Enjoy!