The Echinoid Directory

Key to the major groups of Spatangoida - 2

6a. Apical disc ethmophract
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6b. Apical disc ethmolytic

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7a. Peristome subcentral with ambulacra in grooves on oral surface and with extensive phyllodes
Asterostomatidae

7b. Peristome anterior; ambulacra not forming prominent grooves on oral surface and phyllodes not extensive

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8a. Labral plate stout and subquadrate: extending to third or fourth ambulacral plate. Ambulacra flush aborally
Paleopneustidae

8b. Labral plate short and not extending beyond ambulacral plate 2

Macropneustidae

9a.Sternal plates strongly asymmetric

Toxasteridae
9b. Sternal plates more or less symmetric.
Micrasteridae