An extinct order of primitive irregular sea urchins of the class (Echinoidea), which retain a functioning lantern tern throughout life. Although their periproct opens on the oral surface in the posterior interambulacrum, the test still shows considerable radial symmetry. The mouth is large, circular, and centrally positioned and is indented by sharp buccal notches. The lantern has stout teeth that are wedge-shaped in cross section. Ambulacral plating is trigeminate adorally but usually simple adapically; the tube feet and their pore pairs are undifferentiated. Tubercles are arranged radially on the oral surface, and tuberculation is distinctly finer adapically. The holectypoids are distinguished from other primitive irregular echinoid groups by the presence of a fifth genital plate in the apical disc that may be perforated by a gonopore.