Atrypa oklahomensis brachiopod from the Bois d'Arc formation of Oklahoma

The brachiopod genera Atrypa makes an appearance in the Bois d'Arc formation, as one would expect. It's pretty much an index fossil for the Devonian period as far as I'm concerned. In this case we have Atrypa oklahomensis. The shell is rounded with a very convex brachial valve and a slightly convex to flat pedicle valve. The surface of both valves have regularly spaced ribs and fine concentric growth lines. There is a small fold and sulcus present to some degree on every specimen I have found but this is only expressed at the anterior margin with no hint of continuation farther back in the shell. Perhaps this feature is only found in older, mature specimens? Otherwise the margin where the shells meet is relatively flat. The pedicle valve has a small umbo which extends past the brachial valve. There is little to no interarea visible along the hinge line.

Brachial valve

Anterior

Pedicle valve

Posterior

Profile

The specimens I found are somewhat larger compared to those I have from the Haragan formation.

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.

References: 
"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