The Echinoid Directory

Irregular echinoids with:

• large central peristome with buccal notches;
• fully developed and functioning lantern throughout life;
• perignathic girdle composed of auricles;
• teeth wedge-shaped in cross-section;
• apical disc compact;
• ambulacral plates compound with three elements in each plate in acrosaleniid pattern (i.e. a simple plate alternates with two overlain by the primary ambulacral tubercle); on oral surface one or both smaller elements commonly reduced to demiplates;
• Ambulacral pore-pairs not forming phyllodes adorally;
• tubercles perforate and crenulate;
• spines short, solid, without a cortex.

 Early Jurassic (Pliensbachian) to Late Cretaceous (Maastrichtian); worldwide.

Holectypoids are primitive irregular echinoids that possess a functional jaw apparatus and have a large central peristome. The periproct opens orally in Holectypina, but aborally in Pygasterina. Cassiduloids differ in having a much smaller peristome and have pore-pairs crowded adorally to form phyllodes.

Duncan, P. M. 1889. A revision of the genera and great groups of the Echinoidea. Journal of the Linnean Society, Zoology 23: 1-311.