Cidaroida

Their primary spines are much more widely separated than in other sea urchins, and they have no buccal slits. Other primitive features include relatively simple plates in the test, and the ambulacral plates continuing as a series across the membrane that surrounds the mouth. ==Families==

According to World Register of Marine Species: • family Anisocidaridae Vadet, 1999 † • superfamily Cidaroidea Gray, 1825 • family Cidaridae Gray, 1825 • family Ctenocidaridae Mortensen, 1928a • family Paurocidaridae Vadet, 1999a † • family Diplocidaridae Gregory, 1900 † • family Heterocidaridae Mortensen, 1934 † • superfamily Histocidaroidea Lambert, 1900 • family Histocidaridae Lambert, 1900 • family Psychocidaridae Ikeda, 1936 • family Miocidaridae Durham & Melville, 1957 † • family Polycidaridae Vadet, 1988 † • family Rhabdocidaridae Lambert, 1900 † • family Serpianotiaridae Hagdorn, 1995 † • family Triadocidaridae Smith, 1994c † Image:Stylocidaris affinis.jpg|Stylocidaris affinis (Cidaridae) Image:Notocidaris.jpg|Notocidaris sp. (Ctenocidaridae) Image:Histocidaris variabilis.jpg|Histocidaris variabilis (Histocidaridae) Image:Tylocidaris ohshimai.JPG|Tylocidaris ohshimai (Psychocidaridae) Image:Miocidaris coaeva MHNT.PAL.2006.94.jpg|Miocidaris coaeva (Miocidaridae, fossil) Image:Paracidaris florigemma.JPG|Paracidaris florigemma (Polycidaridae, fossil) Image:Cidaris blakei.jpg|Cidaris blakei, abyssal Caribbean species Image:Eucidaris tribuloides (Slate-pencil Urchin).jpg|Eucidaris tribuloides, shallow Caribbean species Image:Phyllacanthus.jpg|Phyllacanthus imperialis, shallow Indo-Pacific species Image:Plococidaris verticillata.jpg|Plococidaris verticillata, rare shallow Indo-Pacific species Image:Prionocidaris baculosa.jpg|Prionocidaris baculosa, from Philippines Image:Reef0228.jpg|Chondrocidaris gigantea, from Hawaii ==References==