The Echinoid Directory

 Stem group echinoids with:

• globular to elliptical test composed of ambulacral and interambulacral plates; plates rather thick and sutures vertical
• ambulacral zones biserial to multiserial; no differentiation between oral and aboral surfaces
• no internally perradial flanges to ambulacral plates
• interambulacra composed of 4-20 columns of uniform, hexagonal plates
• no primary tubercles; small secondary spinules only in both ambulacral and interambulacral zones.

Lower Devonian to Lower Carboniferous, USA, Europe, former Soviet Union, China, Australia.

This is a well defined group of closely related taxa that differ primarily in details of their ambulacral plating. The thick, uniformly hexagonal interambulacral plates that lack primary tubercles and which are simply covered in a fine uniform scattering of small granules make this clade readily identifiable.

The following taxon is clearly a member of this family, but is too incompletely known to be classifiable to genus level and is a nomen dubium: Xysteria.