Cidaroida

Cidaroida, also known as pencil urchins, 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 †