The purpose of this note is to describe a newly discovered Middle Pennsylvanian fauna from the Francis Creek shale of the Mazon Creek area, Illinois. For convenience, the well-known terrestrial fauna is designated the Braidwood concretion fauna and the marine fauna will be referred to as the Essex concretion fauna. Most of the Essex fossils are impressions of soft-bodied forms. The fauna includes such unusual fossils as polychaete worms, jellyfish, nudibranchs, holothurians, and chitons. One abundant element is a unique wormlike fossil resembling modern bathypelagic nemerteans in general body form. The assemblage of ecologic types represented in the Essex fauna and its stratigraphic occurrence suggest a near-shore environment of deposition.