I've written about the brachiopod Meristella atoka from the Haragan formation of Oklahoma a few years back, but these next examples come from the Bois d'Arc formation. Both have the typical subtriangular to subpentagonal shape (this means they are roughly shaped like a triangle or pentagon when looking at a valve from the dorsal or ventral side), smooth shell with just a few concentric growth lines and a simple fold and sulcus. The latter feature is highly variable as can be seen in the two specimens below and when one compares the specimen from the Haragan formation linked above.
Specimen #2 - Pedicle valve (with a bit of Limonite/Pyrite coating)
I think this next specimen may be a juvenile specimen of the same species. Pedicle valve
I found the specimens shown on this page in the Bois d'Arc formation
(Cravatt Member) near Clarita, OK which is Devonian in age (Lockhovian stage). The Bois
d'Arc overlies the Haragan formation but both are
the same age (Devonian, Lockhovian stage). They are both
roughly correlative to the Helderberg fauna of New York and thus are
contemporaneous with the Coyemans, Kalkberg and New Scotland formations.
"Stratigraphy and Paleontology of the Hunton Group in the Arbuckle
Mountain Region, Part V - Bois d'Arc Articulate Brachiopods " Amsden,
1958, Oklahoma Geological Survey Bulletin 82