The Echinoid Directory

Key to the major clades of Atelostomata Zittel, 1879

1a. Plastron plating protosternous (labral plate small and followed by undifferentiated biserial series of plates)

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1b. Plastron meridosternous (labral plate followed by a single sternal plate) or amphisternous (labral plate followed by an enlarged pair of sternal plates)

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2a. Apical disc disjunct, with posterior oculars separated from remainder of apical disc by interambulacral plates (grey)

stem group Atelostomata

2b. Apical disc with contiguous plating (posterior ocular plates abut posterior genital plates)

Basal holasteroids

3a. Ocular plates II and IV meet behind the anterior genital plate(s), separating them from the posterior paired genital plates

Holasteroida (Meridosternata)

3b. Ocular plates II and IV exsert so that all four genital plate abut