Cidaroida, also known as pencil urchins,250 Million Years of Bindin Evolution is an order of primitive sea urchins, the only living order of the subclass Perischoechinoidea. All other orders of this subclass, which were even more primitive than the living forms, became extinct during the Mesozoic.
Description 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 †
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