Bothriocidarids are the strangest of all echinoids, and in the early 20th Century there was fierce debate as to whether they really belonged in the class Echinoidea. Thanks to the detailed work of Mannil (1962) their structure is now reasonably well understood. Paul (1967) reinterpreted the test as being composed of entirely ambulacral plates, based on comparison with his new genus Neobothriocidaris. This peculiar structure distinguishes them from all other echinoids.
Mannil, R. 1962. [The morphology and taxonomy of Bothriocidaris (Echinoidea)]. Ensv Teaduste Akadeemia Geologia Instituudi Uurimused 9, 143-190 [in Russian with English summary].
Paul, C. R. C. 1967. New Ordovician Bothriocidaridae from Girvan and a reinterpretation of Bothriocidaris Eichwald. Palaeontology 10, 525-541.