Fell, Howard B. Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts.
An order of Eognasthostomata which exhibits various stages in the backward migration of the anus out of the apical system. They have four genital pores, noncrenulate tubercles, and simple ambulacral plates. All members are referred to a single family, the Pygasteridae. They apparently arose from Triassic Pedinidae and occur in the Jurassic and Cretaceous of the Northern Hemisphere. They were formerly classified with other bilaterally symmetrical echinoids in the artificial assemblage Irregularia. See also: Echinodermata; Irregularia
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