Differs from Echinothrix in ambulacral tuberculation. In Diadema there is only a single primary ambulacral tubercle to each compound plate whereas in Echinothrix the primary tubercle is joined by four to six other tubercles only a little smaller, and the perradial tuberculate band thus appears broad and densely tuberculate.
In the past hollow verticillate diadematoid spines in the fossil record have been assigned to the genus Diadema. However, these are basically indeterminate. Diadema palmeri is distinct within the genus in having a monocylclic apical disc. The species were reviewed by Coppard & Campbell (2006a, b).
Baker, A.N. 1967. Two new echinoids from northern New Zealand, including a new species of Diadema. Transactions of the Royal Society of New Zealand 8(23), 239-245, pls 1-3.
Coppard, S. E. & Campbell, A. C. 2006a. Taxonomic significance of test morphology in the echinoid genera Diadema Gray, 1825 and Echinothrix Peters, 1853 (Echinodermata). Zoosystema 28, 93-112.
Coppard, S. E. & Campbell, A. C. 2006b. Systematic significance of trident pedicellariae in the echinoid genera Diadema and Echinothrix. Invertebrate Biology 125, 363-378.
Gray, J. E. 1825. An Attempt to divide the Echinida, or Sea Eggs, into Natural Families. Annals of Philosophy, new series 10, 423-431.
Mortensen, T. 1940. A monograph of the Echinoidea. Volume III. 1, Aulodonta. C.A. Reitzel, Copenhagen.