Large, semielliptical or subquadrate shell; both valves nearly flat. Surface with strong, regular, closely arranged radial striae, fine concentric growth lines, and some more-or-less concentric wrinkles.
A search of the name Rhytistrophia beckii on the Fossilworks.org website, lists the name as one of several "alternative combinations" along with Leptostrophia beckii and Strophomena (Strophodonta) beckii. In Hall's 1861 "Paleontology of New York" Vol 3 (pg. 191, fig. 22) it is known as Strophodonta beckii.
The shell is embedded in the matrix still so I'm not sure if it's a single valve or has both. The white mark is some damage the fossil took when it tumbled down the cliff face.
This fossil was collected from the Kalkberg formation (Devonian, Pragian to Lockhovian stage) near Schoharie, New York.