Palaeoxyris prendeli
Pennsylvanian
Francis Creek Shale
Pit 11, Will Co.
Illinois, USA.

Palaeoxyris represents fossilized egg cases attributed to ancient cartilaginous fishes (chondrichthyans), relatives of sharks and rays. These structures are among the rare reproductive fossils preserved from Paleozoic marine and marginal environments.
51mm egg case in 57mm half concretion