Here is a nice example of Archimedes sp. from the Warsaw Formation of Illinois. This unusual cork-screw shaped bryozoan commonly referred to as an "Archimedes Screw" is unique to the Mississippian aged rocks of North America.
Bryozoans are microscopic aquatic invertebrates that live in colonies, forming skeletal structures. The form of these skeletal structures is unique to each particular species. The individual Byrozoans which form the colonies are called zooids and are filter feeders, straining nutrients from the surrounding water.