Irregularia is an extant infraclass of sea urchins that first appeared in the Lower Jurassic.

Temporal range: Lower Jurassic–recent
Echinocardium cordatum.jpg
Echinocardium cordatum
Scientific classification e
Kingdom: Animalia
Phylum: Echinodermata
Class: Echinoidea
Subclass: Euechinoidea
Infraclass: Irregularia
Latreille, 1825


Description and characteristicsEdit

These particular sea urchins are distinguished from other sea urchins by their irregular shape: the anus and often even the mouth are no more at the two poles of the test, creating a bilateral symmetry instead of the classical 5-fold symmetry of echinoderms. The group includes the well known heart urchins, as well as flattened sand dollars, sea biscuits and some other forms. Most of them live inside the sediment, moving in thanks to their particular spines, and feed on its organic fraction.


Echinolampas ovalis, Middle Eocene, Civrac-en-Médoc, France.
Echinolampas ovalis, Middle Eocene, Civrac-en-Médoc, France; oral surface.


  • Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. p. 981. ISBN 0-03-056747-5.
  • Kroh A, Mooi R (eds.). "Irregularia". World Echinoidea Database. World Register of Marine Species.