I found a number of these round, nut like brachiopods while searching the Bois d'Arc formation in Oklahoma. They are called Sphaerirhynchia lindenensis and seem to have a bit of variability in their features. Overall the shells are rounded to subpentagonal in outline and rectangular to square in cross section. Both valves are convex and have strong, rounded ribs decorating the surface. The margins of both valves are flat except along the anterior margin where the shell develops "teeth" that correspond to the raised ribs on each valve. These "teeth" alternate with the valve below and interlock when closed. This same pattern is present on the fold and sulcus which is entirely expressed in the anterior margin. The pedicle valve has an umbo that extends past the brachial valve and masks the interarea.
I mentioned before that Sphaerirhynchia lindenensis is variable in appearance. This is expressed in the degree of convexity of the valves and width of the ribs. These variations could represent different species but as yet I have not found any literature that describes the fauna of the Bois d'Arc other than Amsden's original paper.
Specimen #1 - Brachial valve
Specimen #2 - Pedicle valve
Specimen #3 - Brachial valve
I found the specimen 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