When you compare the previous brachiopod I posted about, Rhytistrophia beckii, to today's fossil, Leptaena rhomboidalis there are some key differences. Both are large shells that are semiellipical (half moon shaped) to subquadrate (nearly square) with straight hinge lines. The valves of both shells are nearly flat but the margin of the shell in L. rhomboidalis is turned 90 degrees from the regular shell surface all along the edge. Another distinguishing feature is the somewhat regular, concentric wrinkles or corrugation of the shell which was only weakly expressed in R. beckii. This fossil is more common than R. beckii based on the number of specimens I found.
Here is a specimen of the brachial valve that shows the interior.
These specimens comes from the Kalkberg formation which is Devonian in age (Lockhovian to Pragian stage) and was found in a roadcut near Schoharie, New York.