The Echinoid Directory

 Calycina with:

• apical disc relatively large; monocyclic, with plates very loosely bound to the corona and therefore rarely preserved in place (caducous). Periproct large with periproctal plates partially tesselated into disc (inner margin of apical disc circlet is angular);
• ambulacral plating compound in diadematid or phymosomatid style; trigeminate to polygeminate;
• primary tubercles crenulate; ambulacral and interambulacral tubercles similar in size at ambitus.

Lower Jurassic (Toarcian) to Eocene, Europe, North America, Middle East, North Africa.

Phymosoma

This order includes two families (Emiratiidae and Phymosomatidae) as well as a number of unassigned genera.

Euechinoidea, Acroechinoidea, Calycina,

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.