Liomesaspis sp.
Mazon Creek Biota
Middle Pennsylvanian (Westphalian D)
Francis Creek Shale Member, Carbondale Formation
Will County, Illinois, USA

15mm horseshoe crab negative in concretion. Rarest of the three known species at Mazon Creek. Found only in the Braidwood Fauna so it is probably a freshwater species.

For a paper on the species see (PDF): Liomesaspis from France