Homeomorphy and environmental plasticity also makes it difficult to classify gastropods. The Whitewater Formation of the Upper Ordovician Cincinnatian series is known to contain Holopea and Loxoplocus . Bivalves are the second most abundant and numerous class of the Phylum Mollusca. The most common sea shells found on a beach are these bivalve shells. Bivalves have two valves and are symmetrical between the valves.
Common bivalves are clams, oysters, scallops, and mussels to name a few.
Unlike gastropods, to survive, they need a fresh food and oxygen rich water to cover their gills. Because of this, bivalves do not live on land or occupy anoxic environments. The bivalves of the Whitewater Formation of the Upper Ordovician Cincinnatian Series were predominantly epifaunal filter feeders. Living on top of hard substrate, these had no siphons. Fillibranch gills, a more complex set of gills, were used by these bivalves for feeding and respiration in the high energy environments.