Below is a relatively well preserved Pelecypod shell from an Actinopteria that I found at Arkona. It came from the Hungry Hollow member of the Widder formation which is Devonian (Givetain stage) aged. Pelcypod shells tend to dissolve faster in the rock since they are made of Aragonite so it's rare to find one that is intact or complete and this fossil may be just a mold of the original. This specimen shows some fracturing and warping of the shell on the top left side which may have occurred as the sediment compacted.
Based on this checklist, that I'm using to ID fossils from Arkona,I am going to call this an Actinopteria boydi.