Brachiopods (brack'-i-oh-pods) are marine animals with two shells, an upper one and a lower one. The right and left halves of each shell are mirror images, but the two shells are not exactly alike. The shells may be of lime, phosphate, or a horny substance, and the shells range in size from less than a fourth of an inch to several inches.
Most brachiopods live attached to the sea floor by a fleshy stalk that is an extension of the soft body. Some forms lose the stalk when they become adults and either attach themselves directly to the sea floor or lie loose in the mud or sand. Some have spines that serve as anchors.
Brachiopods are not common in most oceans today, but at times in the past they were the most abundant shellfish and sometimes formed large shell banks, much as oysters do today.
In Illinois, the fossils are especially common and well preserved in the limestones and shales of Mississippian age in the Ohio and Mississippi River bluffs, but you can find them easily in almost any part of the state.