Euproops danae
Pennsylvanian
Francis Creek Shale
Pit 11, Will Co.
Illinois, USA.

Euproops danae is an extinct xiphosuran, part of a lineage closely related to modern horseshoe crabs such as the Atlantic Horseshoe Crab. These animals were bottom dwelling arthropods inhabiting coastal and estuarine environments during the Carboniferous.
Unusually large 32mm x 37mm horseshoe crab on 52mm half concretion.