What is an order of irregular sea urchins, usually having a more or less heart-shaped shell with four or five petal-like ambulacra above. The mouth is edentulous and situated anteriorly, on the under side called?

spatangoidea — An order of irregular sea urchins, usually having a more or less heart-shaped shell with four or five petal-like ambulacra above. The mouth is edentulous and situated anteriorly, on the under side. (Webster's 1913.)