Ambocoelia umbonata is an index fossil for me indicating the rocks layers can be dated to the Middle Devonian. It's distinctive size and shape make it hard to misidentify and it can be quite prolific in certain layers. The specimen below comes from the Silica Shale (Devonian, Givetian stage) and was collected at Sylvania, OH.
Brachial valve - With the foramen (circular opening through which the pedicle muscle extended) visible on the recurved pedicle valve
Here is a small specimen from the Widder Formation near Arkona.
I found this cool illustration from "Revised Taxonomy and Autecology for the Brachiopod Genus Ambocoelia in the Middle and Late Devonian Northern Appalachian Basin (USA)" by James J. Zambito IV and Mena Schemm-Gregory, Journal of Paleontology 87(2):277-288. 2013. It shows what the possible life positions were for Ambocoelia umbonata (view 1) and Ambocoelia gregaria (view 2).