Heart Urchins, a general name to include the sea urchins of the order Spatangoidea (q.v.). These have the mouth on the lower side, and, as a rule, some distance from the centre toward, the anterior margin. The mouth, as a rule, is oval, and has two lips, and is not surrounded by a floscelle as in the allied order the Cassiduloidea. The anus is either also on the lower side or on the posterior margin. The best-known member of the group is the common Heart Urchin (Spatanyus purpureas, O.F.M.), common on some parts of the English coast. Some well-known .chalk fossils, such as Furciaster coranyuinum, also belong to this series.