Pelecypods (PA-19)
Among the most common shell-bearing organisms in present seas, lakes and streams are oysters, clams, mussels, scallops and other pelecypods. This bivalve animal lives between two shells that give protection and support to soft body tissues. Most pelecypods feed by filtering microscopic organisms from the water. They usually lie on the bottom or burrow into the sediment, so that most of the shell is not visible. Pelecypods appeared in the sea more than 500 million year ago, and throughout the ages most have been marine, but some moved into brackish bays and adapted themselves to life in fresh water. Pelecypod shells may be as small as a pinhead or very large. One valve in most shells is a mirror image of the other, but different environmental conditions may cause the two valves to assume radically different shapes and sizes. Most fossil pelecypods are impressions or molds produced by the shell in the soft sediments. Fossil pelecypods are not especially common in Indiana bedrock.

Our Hoosier State Beneath Us: Paleontology