title: Holasteroida
text: # 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 †
  