Holasteroida
# Holasteroida

Holasteroida is an order of irregular sea urchins.

## Characteristics

These irregular sea urchins are characterized by a particularly marked bilateral symmetry, including for the apical system, which is highly elongated. In some contemporary abyssal groups such as Pourtalesiidae, some species are even bottle-shaped. The mouth (peristome) does not contain an Aristotle's lantern. The anus (periproct) has migrated towards the periphery of the test. The plastron is never amphisternous.

* Fossile of Hemipneustes pyrenaicus (Hemipneustidae, Maastrichtian)
* Echinosigra amphora (Pourtalesiidae)

This order seems to have appeared at the lower Cretaceous.

## List of families

According to World Register of Marine Species :

* Family Hemipneustidae (Lambert, 1917) †
   * genus Hemipneustes L. Agassiz, 1835 †
   * genus Medjesia Jeffery, 1997 †
   * genus Opisopneustes Gauthier, 1889 †
   * genus Plesiohemipneustes Smith & Wright, 2003 †
   * genus Toxopatagus Pomel, 1883 †
* Suborder Meridosternata (Lovén, 1883)
   * Infraorder Cardiasterina †
      * Family Cardiasteridae Lambert, 1917 †
      * Family Stegasteridae Lambert, 1917f †
   * Family Echinocorythidae Wright, 1857 †
   * Family Holasteridae Pictet, 1857 †
      * Genus Salvaster Saucède, Dudicourt & Courville, 2012 †
   * Infraorder Urechinina
      * Family Calymnidae Mortensen, 1907
      * Family Carnarechinidae Mironov, 1993
      * Family Corystusidae Foster & Philip, 1978
      * Family Plexechinidae Mooi & David, 1996
      * Family Pourtalesiidae A. Agassiz, 1881
      * Family Urechinidae Duncan, 1889
* Family Pseudholasteridae (Smith & Jeffery, 2000) †
   * genus Eoholaster Solovjev, 1989 †
   * genus Giraliaster Foster & Philip, 1978 †
   * genus Pseudholaster Pomel, 1883 †
   * genus Taphraster Pomel, 1883 †
* Family Stenonasteridae (Lambert, 1922) †
   * genus Stenonaster Lambert, 1922 †
