I found this yesterday during my creek walk. It appears to either be a Coral or Sponge atop a Concretion. It also has the "mud" on top which is fairly common...but it's very hard. It was found between the Upper Tully and Lower Tully formation which are both Limestone, between is black shale. The diameter on top is approx. 10"-12" and the reveal of the side is approx. 6".
Thanks
Greg