The Echinoid Directory

• Test ovate and thin-shelled with no anterior sulcus; depressed in profile.
• Apical disc ethmolytic with four gonopores, but with the four genital plates apparently fused. In any case the madrepores extend to the posterior of the posterior ocular plates; central to slightly posterior.
• Ambulacra all similar; non-petaloid and flush; only the adapicalmost five or so plates with pore-pairs and short cylindrical tube-feet; other plates with microscopic pores only.
• Peristome towards anterior border and distinctly overarched by labrum; wider than long.
• Labral plate elongate and arrow-shaped, tapering to posterior; extending to middle of second ambulacral plate. Shape of sternal plates not known, but presumably triangular. Episternals unknown but not indented to rear by ambulacral plates.
• Periproct marginal.
• No fascioles.
• Aboral plates with scattered primary tubercles over ambulacral and interambulacral plates. Sternal plates tuberculate. No tubercles on oral posterior ambulacral zones.
• Ophicephalous pedicellariae with distinctive globular head with basal part of valves highly developed and blades reduced.

Recent, East Pacific (Hawaii and Cocos Islands); deep-sea.

Phrissocystis aculeata A. Agassiz, 1904, p. 187 by original designation.

• A. aculeata (Agassiz, 1898); Recent, Cocos Islands, East Pacific.
• A. multispina (Agassiz & Clark, 1907); Recnet, Hawaii Islands.

Spatangoida, Micrasterina, ?Macropneustidae.

?Subjective junior synonym of Argopatagus Agassiz, 1881.

Argopatagus and Phrissocystis are similar in all respects other than that Argopatagus has a subanal fasciole whereas Phrissocystis does not. Mortensen (1950) believed that the two should be synonymized. Distinguished from Archaeopneustes by having ambulacral plates tall and hexagonal over the upper surface and with double pores only developed adapically. In Archaeopneustes the aboral ambulacral plates are wide and low and have pore-pairs almost to the ambitus.

Agassiz, A. 1898. Reports on the dredging operations of the west coast of Central America to the Galapagos; part 23, Preliminary reports on the Echini. Bulletin of the Museum of Comparative Zoology, Harvard College, 32, 7-86, pls 1-13.

Mortensen, T. 1950. A monograph of the Echinoidea. V, Spatangoida 1. C. A. Reitzel, Copenhagen.