From Ohio History Central
Clams, sometimes called pelycopods (Class Pelecypoda) are filter-feeding animals with two identical shell halves. Some burrowed in bottom mud, whereas others rested on the sea bottom and a few were capable of swimming. Marine species are abundant and diverse in Ordovician through Permian rocks in Ohio and are commonly well preserved. Pennsylvanian and Permian rocks yield species that lived in fresh or brackish waters. Clams are common in some sediments deposited in lakes or streams during the latter part of the Pleistocene Ice Age.
Particularly interesting to fossil collectors are the large clams found in Middle Silurian rocks that are called