Pictured here are some Pennsylvanian bivalves in limestone, collected near Bonner Springs, Kansas. (By Pennsylvanian, we mean the Pennsylvanian Period 323 – 290 million years ago.) But what is a bivalve? Bivalves have hard shells with two parts called valves and are usually bilaterally symmetrical, which means that their left and right sides look the same. Bivalves belong to the Kingdom Animalia, Phylum Mollusca, Class Bivalvia, and include clams, oysters, scallops, and mussels. Bivalve fossils date back all the way to the Cambrian Period, 510 million years ago.
If you want to find one, Kansas is a pretty good place to start looking. Kansas has a lot of limestone and shale which is chock full of fossilized clams and oysters. You might find a little one like in the photo above, or you could find an inoceramid clam, some of which were up to 6 feet in diameter. Inoceramid clams are extinct, but they used to live on the ocean floor around western Kansas during the Cretaceous Period about 145 to 65 million years ago (yes, the Midwest used to be underwater). This one is about a foot long and you can see that the back is covered in oysters
Sources: Geo Kansas: http://www.kgs.ku.edu/Extension/fossils/bivalve.html Oceans of Kansas http://oceansofkansas.com/Inoceramids.html