The Echinoid Directory
Key to the major groups of cassiduloid, stem group Neognathostomata and stem group Clypeasteroida
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1a. Ambulacral pores double on each plate.
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1b. Ambulacral pores single on plates beneath the petals.
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2a. Apical disc with Genital plate 3 always imperforate (two or three gonopores present).
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2b. Genital plate 3 perforated by a gonopore (three or four gonopores).
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3a. Ambulacra with inner and outer series of pore-pairs in phyllode, the inner series on occluded plates.
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3b. Ambulacra with single irregular series of pores in phyllode; plating simple or with small adradial pyrinoid elements.
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4a. Periproct supramarginal, in anal sulcus.
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4b. Periproct inframarginal to oral; flush.
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5a. Lantern and perignathic girdle absent in adults.
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5b. Lantern and perignathic girdle present in adults.
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6a. Demiplates superficial; not penetrating through to interior of ambulacrum.
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6b. Demiplates a fully integral part of the ambulacral zones.
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