Here is a tiny brachiopod that I picked up off E-bay called Lissostrophia coooperi. It comes from the Henryhouse formation near Ada, Oklahoma. The original label indicated it was Productella sp. but a search through the texts could not turn up anything like that from the formation. I finally traced the identity to Bulletin 78 from the Oklahoma Geological Survey: "Stratigraphy and paleontology of the Hunton Group in the Arbuckle Mountain Region"pages 77-78, plate 12.
A group of the fossils in my hand
The Henryhouse formation is Silurian in age and often referred to as "Niagaran" which would roughly correspond to the Aeronian thru Gorstian stages of the ICS (439-421 mya).
Here is a reference , by Thomas Amsden, that describes Lissostrophia cooperi from The Journal of The Washington Academy of Sciences, vol. 39:pages 202-203,(1949)