Lovenechinus lacazei (Julien) (most likely this species but I'm not sure if there's really enough diagnostic detail).
Lower Carboniferous, probably Tournaisian.
Very rare anyway but of especial interest as it is from the Jurassic Doulting Stone (Bajocian, Inferior Oolite) of Somerset, UK. This is a limestone full of Carboniferous detritus, formed when the Jurassic sea was washing up against the Mendip Hills Carboniferous high ground.
(Just acquired via a dealer from an old collection that included Carboniferous coral and crinoid fragments from the same location. No other echinoids though!)
2.3cm across