The Illinois State Geologic Survey (ISGS) has a page on the trilobites of Illinois. It has good information about time periods and has a few pictures of the species that can be found here in Illinois. There's even a paragraph on it's body structure.
Trilobites are so named because the segments on their upper (dorsal) surface usually possess longitudinal furrows that form a three (tri-) lobed division of the body. The central lobe is called the axial lobe, and the two lateral counterparts are called pleural lobes. The dorsal surface consisted of a hard, mineralized protective shield called a carapace; it is this part of the shell, or exoskeleton, that is most commonly preserved in the fossil record. The lower (ventral) surface bore a pair of antennae and numerous pairs of jointed appendages that served as walking, swimming, feeding, and respiratory organs. The ventral surface, however, consisted of relatively soft tissue and rarely is preserved. A typical trilobite is about 2 inches long, but some are less than half an inch in length and giants of the group measure fully 2 feet.
