I wrote this a little while back:
"Sometimes there’s a protruding spike sort of thing on a turtle shell, just as in the image above. This has nagged my thoughts for years (meaning that you don't often see this detail on every box turtle you come across, both in person and in other people's images that I lift and use here on this blog)..."
And just now I took a look at another box turtle photo and said:
"Oh! There's a little spike at the nuchal notch of this one. "