Small order of echinoderms which have five double columns of perforate plates and five columns of single imperforate plates. Traditionally they were regarded as the ancestral stock of all echinoids, but this is now considered unlikely. There are only three genera, Bothriocidaris (the earliest echinoid), Neobothriocidaris, and Unibothriocidaris, ranging in age from Middle Ordovician to Upper Silurian.