The Echinoid Directory

 Stem group echinoids with:

• test composed of ambulacral and interambulacral plates; plating more or less firmly sutured
• ambulacral zones multiserial; pore-pairs arranged into discrete columns; pore-pairs small and undifferentiated
• internally perradial flanges on ambulacral plate enclose the radial water vessel
• interambulacra composed of a single column of plates only
• small mamelonate tubercles on both ambulacral and interambulacral plates.

Lower Carboniferous (Visean) to Permian, USA, Europe.

Unfortunately, neither genus assigned to this family is particularly well known. However, the unique uniserial nature of the interambulacra distinguishes it from all others. The enclosed nature of the radial water vessel (known only for Xenechinus) suggests that the group is primitive.