Sea urchins are free-living, spherical or sometimes flattened echinoderms. To an extent, they resemble 'halved tennis balls with spines'. Hemipneustes is rightly known as the echinoid of the Limburg Cretaceous. It is not too difficult to collect a well-preserved specimen in one of the quarries (especially there where the Nekum Chalk is accessible). Hemipneustes grew to a size of some 10 cm and lived mostly partially buried in the sea floor. The lower portion thus stood a better chance of being preserved than the upper half: a 'beheaded' Hemipneustes is not at all rare. Another common species is Hemiaster, a c. 2 m large echinoid which is especially found in the unit known as the Meerssen Chalk