I'm no good at fossils!
I don't have a clue. 
I can tell you that it's about 320 million years old. I know that because I recently asked a paleontologist what age the fossils in the area are. It's from Ayrshire. It was found among round crinoid stems and lots of shell fossils. The same expert told me that these fossils are older than the dinosaurs. She said if I find a dinosaur to give her a call because it's theoretically impossible. 
Oh by the way: Scotland had dinosaurs. 
This circular fossil has quartz or calcite in the circle part. I guess whatever creature it was fell in a sandbank and died, only for years of dripping gunk to calcify in it.