Distinguished from miocidarids by having a fully rigid corona with vertically abutting sutures, and from Rhabdocidaridae by its simple ambulacral plating, with uniform tuberculation. Primary tubercles may retain weak crenulation, but the platform is never strongly crenulate as is the case in Rhabdocidaridae. Histocidaridae have strongly crenulate tuberculation in contrast to Cidaridae, and Polycidaridae have heterogeneous ambulacral tuberculation. Psychocidaridae have massive mamelons that are imperforate or bear just a small, rudimentary opening, and their spines are typically clavate. Ctenocidaridae differ in lacking clear scrobicular cicles around the areoles of primary tubercles. Although Plegiocidaris has distinctly crenulate tubercles throughout, it has almost competely uniform ambulacral tuberculation and its dense and uniform extrascrobicular tuberculation suggests affinties with Cidaridae. It is here tentatively treated as a stem group Cidaridae.
The following taxa are based on spines and indeterminate to genus: Palmerius Cutress, 1980; Romanocidaris Vadet, 1991; Clavicula Smith, 1817.
Gray, J. E. 1825. An Attempt to divide the Echinida, or Sea Eggs, into Natural Families. Annals of Philosophy, new series 10: 423-431.