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Oepikina minnesotensis. Occurs mainly in upper Platteville formation. I'm not 100% sure if they occurs in decorah formation as I had found unusual circular inflated shells but its much harder to see their hinge line or muscle scars at present. Either the decorah version is a extreme version of Doleriodes pervetus or Oepikina minnesotensis. They usually differs from Strophomena filitexta into being more compact and rounder, also more inflated. They're much smaller compare to Oepikina inquassa and less variable in shapes. You can see the fossil in the farthest left of the picture have unusual well preserved muscle scar.