Its definitely a tooth (the pore system you are seeing is the 'Bradyodont' structure) I have found many teeth from seven genera of Bradyodont sharks at that location. Most I can ID myself, this one, however, seems to match the drawing and description of Xystrodus given in Worthen's "Geographic survey of Illinois - book 6". Thank you for the reply.
Yes - my ID was pretty uncertain so I took the liberty to pass it onto someone who does know about Lower Carboniferous fossils - he ID'd it as a palatal fish tooth. I think he was going to look up the species so I'll update you if he does.