Echinoids are cool critters that come in two basic flavors, regular and irregular. The regular echioids are the ones you see in aquariums and on nature shows with the long spines crawling along the bottom of the reef. Irregular echinoids are more often partially submerged in the sediment and do not have large showy spines. Hardouinia kellumi is an example of an irregular urchin and could also be known as a sea biscuit. The specimen below is from the Cretaceous ages sediments at Rocky Point Quarry in North Carolina. It comes from the Peedee formation which is dated to the late Maastrichtian (66-67 mya)
Adaptical surface
Anterior
Left profile
Posterior
Right profile
Adoral surface
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