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Terrapene carolina triunguis
Yes, that is a three-toed box turtle. Most do have 3 toes on their hind feet, but some do have four. This sub-species often has a very plain brown shell. Based on your spotting location, I'm pretty sure it has to be a three-toed. They are very common in Missouri. In fact, it is the official state reptile of our state.
I think that's a Three-Toed Box Turtle. Threw me at first because I was looking at the FRONT feet. Does it have three toes on its rear feet?
Hmmm, sure looks like a box turtle, but I've never seen one with a shell like that. Interesting.