This large steinkern, or cast, fossil is a pelecypod that I found at the Cole Hill locality in Madison County, NY. I've been reviewing some resources like "The Paleontology of New York" (James Hall, 1884) and "A field Guide to the Devonian Fossils of New York" (Karl Wilson, 2014) and I think that this fossil is of a Actinodesma erectum. It's a large, oval shaped steinkern with the body vertically aligned (or erect which is where it gets it name) which is unlike many other genera that are diagonally or horizontally angled. It would be nice if I'd found this piece with the accompanying mold from either side as that would have more detail. As it is, the fossil does not have the hinge line preserved which should be shorted than the body length with a wing on the right side and a short ear on the left side.
What does the fossil have that helps with the ID? Well, as I mentioned before, the body is vertically aligned and longer than wide as well as having an umbo that narrows slowly.
Due to the size and shape of the specimen I am keeping my initial ID of Actinodesma erectum. Another possible match could be Mytilarca oviformis, but that species has a much shorter hinge line, both valves are convex to the same degree as the other and the umbo is pointed. Actinodesma erectum (Conrad) Hall 1877 is the accepted name for this fossil which formerly was known as Avicula erecta Conrad 1842 & Glyptodesma erectum Hall 1883. Thanks to Karl Wilson for the taxonomy history lesson.