The Echinoid Directory

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Key to the orders of Calycina 1a. Apical disc firmly attached to the corona, with one or more enlarged periproctal plates firmly bound to the anterior genital plates. Ambulacral tubercles much smaller than interambulacral tubercles. Salenioida 1b. Apical disc large and pentagonal, plating generally lost in fossils. Ambulacral and interambulacral tubercles similar in size. Phymosomatoida