I looked back through my blog posts and am surprised that I have not yet featured this next fossil. It's Grammysia bisulcata and this specimen comes from the Windom shale. It's a single right valve which shows the oval shape well. The other defining features are the widely spaced, concentric ridges and a wide furrow that extends from the umbo to the margin. It's this furrow that distinguishes this species from Grammysioidea arcuata.
I thought I'd found specimens of this species in my local Mahantango formation rocks but I can't find a blog post for it. I will have to dig through my boxes and see if I can't rectify that.
This specimen was found at a borrow pit on Deep Springs Road in Madison County, NY which exposes the Windom shale member of the Moscow formation (middle Devonian in age, Givetian stage.)