The Echinoid Directory

Key to genera of primitive irregular echinoids

1a. Single large primary tubercles present on intermbulacral plates.
1b. Distinct primary tubercles absent.
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2a. Periproct enclosed by apical disc.
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2b. Periproct separated from apical disc.
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3a. Anal sulcus present.
3b. No anal sulcus.

4a. Periproct inframarginal.
4b. Periproct marginal to supra-ambital
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6a. Apical disc elongate with small auxilliary plates incorporated between paired oculars and posterior genital plates.
6b. Apical central, tetrabasal with 4 gonopores. Genital plate 2 separates posterior genital plates. Two small auxilliary plates incorporated between posterior oculars.