The Echinoid Directory
Key to the major clades of Atelostomata Zittel, 1879
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1a. Plastron plating protosternous (labral plate small and followed by undifferentiated biserial series of plates) |
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) |
2b. Apical disc with contiguous plating (posterior ocular plates abut posterior genital plates) |
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3a. Ocular plates II and IV meet behind the anterior genital plate(s), separating them from the posterior paired genital plates |
3b. Ocular plates II and IV exsert so that all four genital plate abut |