A more restricted usage of the family is applied here, since many of the taxa included here by Kier (1966) are treated as a separate clade, the Proterocidaridae. Proterocidaridae differ from Echinocystitidae in having large, circular, oral pore-pairs of very distinctive form. In addition their radial water vessel is generally not enclosed by perradial extensions of the ambulacral plates. Echinocystitidae are presumably a plesiomorphic sister group to Proterocidaridae, but a modern analysis of the problem is required.
Kier, P. M. 1966. Noncidaroid Paleozoic echinoids. Pp. U298-U312 in Moore, R. C. (ed.) Treatise on Invertebrate Paleontology. University of Kansas Press & Geological Society of America, Boulder.