Distinguished from Arbacioida by their apical disc, which is much larger and not firmly connected to the corona. This feature also distinguishes them from most Salenioida except for some of the basal acrosaleniids. However, acrosaleniids differ from phymosomatoids in having ambulacral tubercles much smaller than interambulacral tubercles at the ambitus. Temnopleuroids can have a similar tuberculation, but never have a monocyclic apical disc. Some Diadematoida are very similar in test morphology, but they have grooved teeth, not keeled teeth.
Mortensen, T. 1904. The Danish Expedition to Siam 1899-1900: II, Echinoidea. Kongl. Dansk. Vidensk. Selsk. Skr. (7) 1, 1-124, pls 1-7.