Could someone identify this for me please? It's about 9 mm across, found in chalk at Windsor Hill, Bucks (grid ref SP826027). According to the geology map it was in the 'Holywell Nodular Chalk formation/New Pit Chalk formation'. It looks like it ought to belong to a shark, but any thoughts much appreciated.
It was on a nature reserve where they had dug out a small area of chalk last winter. I'd been over it several times and found urchin spines, brachiopods etc, but somehow missed this until the last visit a few days ago. Unfortunately most of the chalk has now been broken down by frost, so I'm not expecting to find any more.