Trilobites (try'-low-bites) have been extinct for about 250 million years. They commonly are preserved in great detail and are prized as fossils. Two grooves extending down the back of the animal divide it into three lobes; hence, the name "trilobite".
Trilobites had a head with eyes and a mouth, a jointed body, and a tail. The animals were cousins of crabs and lobsters and lived in the sea.
They were covered with a horny armor, jointed so that the animal could move. Trilobites shed their armor much as snakes shed their skins, so each animal could have provided several fossils.
Trilobites can be found in northeastern Illinois as well as the western part of the state.