The Echinoid Directory

 Hemiasterid spatangoids with:

• apical disc ethmolytic with two gonopores (gonopores absent from anterior genital plates);
• labral plate elongate, extending to second or third ambulacral plate;
• sternal plates symmetrical; episternal plates offset;
• peripetalous fasciole present; indented by one plate behind the anterior petals;
• anterior interambulacra amphiplacous, lateral interambulacra meridosternous;
• ambulacral zones not strongly pinched as they cross the peripetalous fasciole.

Eocene to Recent; worldwide.

Mortensen (1950) included only Palaeostoma in the family Palaeostomidae, remarking on the uniqueness of its pentagonal peristome and five triangular plates covering the opening. Ditremaster has a similar pentagonal peristome as juveniles and a similar ethmolytic apical disc with only two gonopores. Both genera further differ from hemiasterids by having the peripetalous fasciole indented by a single interambulacral plate behind the anterior petals. One other genus, Sarsiaster, is also included, based on its similar apical disc and peripetalous fasicole pattern, even though it has a kidney-shaped peristome.

The taxon is treated as a subfamily of Hemiasteridae.

Trachyaster has an ethmolytic apical disc and may belong here, although it retains four gonopores.

Loven, S. 1868. Om Leskia mirabilis Gray. Ofversigt af Kongelige Vetenskaps-Akademiens, Forhandlingar 24 (1867), 431-440.