Fossils - NOT amateur radio

We have walked over tiles in the Grand Arcade in Cambridge for years. Only in the last few months have we have noticed the fossils in them. Now when we look down there are hundreds!  

How can we live here for so many years and not see these?!! I bet if we missed these, many others have too.

I think this is an ammonite millions of years old.

See https://en.wikipedia.org/wiki/Ammonoidea .