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Holectypoida (subphylum Echinozoa, class Echinoidea) An order of irregular echinoids in which the ambulacra are narrower than the interambulacra, there are lateral flanges on the teeth, and the periproct is usually on the lower (sometimes the posterior) surface. They first appeared in the Lower Jurassic, and diversified in the Cretaceous, but only two genera (Echinoneus and Micropetalon) have survived.