'Cidaris' strombecki (Desor, 1858)
The highly distinctive, large and club-shaped primary spines of this cidaroid are very scarce and restricted to the Grey Chalk Subgroup. No associated test has ever been found and the true affinities of this species are unknown.
1). Isolated primary spine of 'Cidaris' strombecki (x2.5, Grey Chalk, Clayton, Sussex, Willett Collection, Booth Museum, BMB 007867, by kind permission of John Cooper).