The Echinoid Directory

• Test delicate with flexible plating throughout; ambitus rounded.
• No conspicuous difference between upper and lower surfaces.
• Apical disc monocyclic; plating reduced but with genital plates.
• Ambulacra trigeminate, On aboral surface the large central element bearing a primary tubercle does not reach the adradial suture. Instead one of the demiplates forms an adradial row.
• Primary tubercles perforate and non-crenulate; developed ambitally. Areoles are not sunken. Adapically there is a primary tubercle on every other plate.
• Sphaeridia present on only one demiplate on oral surface.
• Aboral spines ending in a simple point; in life bearing enlarged fleshy sacs.

Recent, Indonesia.

Phormosoma alternans de Meijere, 1902, p. 2, by original designation.

• Only the type species.

Euechinoidea; Echinothurioida; incertae sedis (?sister group tothe Echinothuriidae).

Monotypic.

Differs from other echinothurioids in having a continuous adradial series of demiplates aborally, with the large element completely occluded from the adradial suture. The lack of a hyaline hoof at the end of oral spines distinguishes this taxon from Echinothuriidae, while the lack of large adoral tubercles with sunken areoles and sphaeridia on just one of the two accessory plates in each triad distinguish it from the Phormosomatidae. Mortensen (1935) placed this taxon in the Phormosomatidae on the basis that it lacked spines with a hyaline hoof, but this is a plesiomorphic feature. It is probably sister group to the Echinothuriidae.

Mortensen, T. 1935. A monograph of the Echinoidea. Volume 2, Bothriocidaroida, Melonechinoida, Lepidocentroida and Stirodonta. C.A. Reitzel, Copenhagen.