The Echinoid Directory

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Key to major clades of Holasteroida

1a. Plastron plating protosternous at least in juveniles (may become quasi-metasternous). Basal holasteroids	1b. Plastron plating either meridosternous or metasternous - i.e. labral plate followed by a single sternal plate. Go to 2
2a. Episternal and subsequent plates biserially arranged. Meridosternata	2b. Plastron plating metasternous. Cardiasteridae