As I was sorting through my Devonochonetes sp. fossils, to find suitable examples to be photographed for this blog, I found one that didn't fit with the rest. It turns out this is my first example of Tropidoleptus carinatus from Arkona. It's a small shell at only 3-4mm wide so I'm not surprised I mistook it for a Devonochonetes at first.
What gives it away are the concentric growth lines near the edge and the wider, thicker radial ribs. Since I didn't recognize this when it was first collected I don't know for sure which formation it comes from. Based on this checklist from 1958 though, Tropidoleptus carinatus is only known from the Hungry Hollow member of the Widder formation.