distinct bilateral symmetry along the plane III-5, with mouth displaced anteriorly on the lower surface and the periproct marginal to submarginal
compact apical disc with ocular plates II and IV separated
posterior interambulacrum on oral surface differentiated to form labrum
one plate in each ambulacral zone bordering the mouth with two tube-feet (and pores).
Lower Cretaceous (Valanginian) to Recent; worldwide
Spatangoids are one of the most diverse of extant echinoid orders. They share with holasteroids the differentiation of the posterior interambulacrum into a plastron and the presence of two tube-feet on one of the plates bordering the peristome in each ambulacral zone. The two groups are easily distinguished however, by their apical disc structure. In spatangoids ocular plates II and IV are small and separated from one another by genital plates, whereas in holasteroids ocular plates II and IV are large and abut.
The following genera remain too poorly known to place at present into any family: