Carboniferous Period is between those periods - http://en.wikipedia.org/wiki/Geological_columnMiddle Ordovician to Late Permian seas
teabag said:We have a similar photo from a trip into White Scar, also concluded it was a coral.
Les W said:Looking at the picture I wonder if it is squashed or just eroded along the long axis at an angle.
I vote Rugose Coral as wellLes W said:Looks like it might be a rugose coral to me.