Hirudocidaris is a sea urchin that is rarely found with perfect preservation. This specimen also preserves remnants of primary spines and quite a few secondary spines, all of which show fine surface detail. Sometimes good results can be achieved with KOH treatment (Potassium Hydroxide). The problem with a fossil like this, however, is that the spines would be blown off and it is not uncommon for the corona to disintegrate into its individual plates. 

To expose the fossil completely including all spines without risk, a mechanical approach is safer. However, since the surface details must not be damaged, the preparation must be done carefully and gently. In this case the preparation could be done very well using fine air abrasives. 

Fossil: Hirudocidaris pyrenaica COTTEAU, 1862

Formation: Lower Cretaceous, Apt

Diameter: 3cm (1.2 inches)

Work required: 9 hours