Basal Meridosternata with:
- apical disc of standard holasteroid structure, with four gonopores;
- frontal groove present from ambitus to peristome as shallow furrow; usually also extending adapically;
- plastron plating meridosternous with sternal plate similar in size to episternal plates and with episternals biserially offset. In some taxa episternals reaching opposite adradial suture making plastron metasternous;
- pores in anterior ambulacrum small and differentiated from those in the paired ambulacra aborally.