The last, identifiable, fossil that I found during my brief visit to the roadcut in Oklahoma that exposed the Viola formation, is the brachiopod Glyptorthis costella. It has a subrectangular shape with valves that are equally convex and decorated with many fine, closely spaced striae. The margin of the valves is flat with no obvious fold/sulcus structure. The posterior of the shell has a nearly flat, wide interarea. Both valves are recurved in the umbo with the brachial valve moreso than the pedicle valve.
This is a loose pedicle valve that I found.
The interior of the shell is nicely preserved with the muscle scars
These specimens comes from the Viola formation which is upper Ordovician (Sandbian to Katian stage) in age and overlies (unconformably?) the Bromide formation. I found them in a road cut along US 177, south of Sulphur, Oklahoma.