The Echinoid Directory

 Irregular echinoids with:

• strong bilateral symmetry; often cordate in outline;
• apical disc elongate with ocular plates II and IV in contact behind the anterior genital plates (true in all but one genus);
• lantern and perignathic girdle absent;
• peristome opening towards anterior border; posterior interambulacrum differentiated to form plastron;
• plastron plating protosternous, meridosternous or metasternous; never amphisternous.

Early Cretaceous (Berriasian) to Recent; worldwide.

 Holasteroida are distinguished from their sister group the Spatangoida by their apical disc structure and by their plastron architecture. Spatangoids have an amphisternous or protoamphisternous plastron in which the first pair of plates following the labral plate are distinctly enlarged, and their apical disc never has oculars II and IV meeting behind the anterior genital plates.

The following taxon is too poorly known to be placed with confidence within any holasteroid grouping:

Corechinus Kongiel, 1936; Upper Cretaceous, Poland.

Titanaster Szorenyi, in Mortensen, 1950; Eocene, Hungary