- Test subcircular in outline with posterior face truncate; wedge-shaped in profile.
- Apical disc ethmophract with four gonopores; central.
- Ambulacrum III weakly sunken adapically; narrow, with large pore-pairs associated with funnel-building tube-feet. Perradial zone narrow.
- Paired ambulacra strongly petaloid; petals bowed; closed distally; with slit-like pores and narrow perradial zone. Posterior petals only slightly shorter than anterior petals in type species.
- Ambulacral plates strongly pinched immediately beneath the petals as they cross the peripetalous fasciole.
- Peristome ovate or weakly kidney-shaped and facing downwards.
- Labral plate longer than wide and flask-shaped; extending to second ambulacral plate. Sternal plates large, symmetrical and forming most of oral surface. Episternals biserially offset; not indented by ambulacral plates.
- Lateral interambulacra amphiplacous.
- Periproct small, situated towards top of posterior truncate face.
- Enlarged subanal pore-pairs present.
- Peripetalous fasciole present; indented by one interambulacral plate behind anterior petals.
- Aboral tubercles small, uniform, not sunken.
Upper Cretaceous (Maastrichtian) to Recent, Madagascar, Pacific, Atlantic.
Hemiaster hickmanni Koehler, 1914, p. 142, by original designation.
- H. expergitus (Loven, 1874); Recent, Atlantic.
- H. gibbosus (Agassiz, 1879); Recent, West Pacific.
- H. hickmanni (Koehler, 1914); Recent, Indian Ocean.
- H. hawkinsi ; Maastrichtian-Danian, Madagascar.
- H. vadosus (Gregory, 1891); Miocene, Malta.
|Classification and/or Status
Spatangoida, Hemiasterina, Hemiasteridae.
Differs from Hemiaster in having very well-developed, broad petals, symmetrical to subsymmetrical sternum and pinched zone in the ambulacra immediately beneath the petals. Proraster also has highly pinched ambulacra beneath the petals, but Holanthus differs from that taxon in having a subcentral apical disc and narrower frontal ambulacrum.
Although Lambert & Thiery (1924) included a small number of Upper Cretaceous forms it is not clear that these are more closely related to Holanthus than to Mecaster. Here I include in this genus all Hemiaster species sensu lato that have highly pinched ambulacral zones immediately beneath the petals.
Koehler, R. 1914. An account of the Echinoidea. Echinodermata of the Indian Museum, part 8, Echinoidea (1). Calcutta, Trustees of the Indian Museum.