Distinguished from Fibularia and its relatives by having internal radial buttresses. Sismondia differs in having branched internal supports and more projecting basicoronal interambulacral plates. The Recent E. provectus de Meijere, 1902 has branched internal supports and is probably a juvenile Peronella. Mortensen (1948) recognized 18 extant species and there are in addition a number of fossil species of uncertain status. The oldest species based on excellent material is E. bisexus Kier from the Middle Eocene of Georgia, USA, described by P.M. Kier (1968). Mironov & Sagaidachny (1984) give a detailed account of the modern species. Zachos (2005) has discussed the distribution of this taxon in southeastern USA during the Eocene.
Mironov & Sagaidachny (1984) distinguished the monotypic subgenus Lepidocyamus on the basis of it having a small number of large periproctal plates and "squamiform projections along the peristome".
- Mortensen, T. 1948. A Monograph of the Echinoidea IV.2 Clypeasteroida. C. A. Reitzel, Copenhagen.
J. W. Durham 1955 Classification of clypeasteroid echinoids. University of California Publications in Geological Sciences 31, 73-198.
Kier, P. M. 1968 Echinoids from the Middle Eocene Lake City Formation of Georgia. Smithsonian Miscellaneous Collections 153(2), 1-45, pls 1-10.
Nisiyama, S. 1968. The echinoid fauna from Japan and adjacent regions part II. Palaeontological Society of Japan Special Papers 13, 1-491, pls 1-30.
Mironov, A. N. & Sagaidachny, A. Y. 1984. Morphology and distribution of the Recent echinoids of the genus Echinocyamus (Echinoidea: Fibulariidae). Transactions of the P. P. Shirshov Institute of Oceanology 119, 179-204, 2 pls [in Russian].
Zachos, L.G. 2005. Eocene dispersal of the echinoid genus Echinocyamus in the southeastern United States. Southeastern Geology 43(4), 215-227