Among the largest brachiopods know is Megastrophia concava which is somewhat common in lower to middle Devonian aged rocks. The shell is typically oval to rectangular in shape, has a convex pedicle valve, a convex brachial valve and a straight hinge line. It is a member of the order Strophomenida and is related to Strophomena, Leptanea, and others with a similar shell morphology. This specimen is preserved by chert/beekite and only a partial internal/external cast of the pedicle valve is visible.
I found this specimen near Lousiville, KY and believe it comes from the Jeffersonville Limestone, dated to the Devonian period (Eifelian stage).