The Echinoid Directory

• Test ovate; thin-shelled. Base somewhat concave towards peristome. Conical in profile.
• Apical disc with four gonopores. Hydropores may expand over adjacent anterior plates.
• Ambulacra all similar with minute simple pores only; all flush.
• Peristome subcentral and sunken in a distinct pentagonal vestibule; subcircular in outline.
• Plastron plating meridosternous with highly reduced labral and sternal plate; subsequent plating biserial.
• Scattered primary tubercles uniformly distributed over the entire aboral surface.
• Periproct on short posterior face. Large; occupying most of the posterior truncate face. Opening between plates Ia8b/9a and 11b/12a.
• No fascioles present.

Recent; west Pacific and Antarctic.

Cystechinus rathbuni A. Agassiz, 1898, p. 79, by original designation

• P. rathbuni (Agassiz; 1898); Recent, Gulf of Panama.
• P. belyaevi Mirnov, 1975; Recent; Antarctic.
• P. vesica (Agassiz, 1898); Recent, Chile, Antarctic.

Holasteroida, ?Meridosternata, ?Urechinina, ?Urechinidae.

Monophyletic.

Differs from Urechinus and other Urechinina in having a marginal periproct and in having the posterior interambulacrum hardly differentiated and, most distinctly, by having the first two plates in all interambulacral zones highly reduced. It also differs in having a distinctly invaginated peristome. Mooi & David (1996) provide a detailed analysis of this genus. The three species vary only in slight details of pedicellarial structure and may well prove conspecific.

Mooi, R. & David, B. 1996. Phylogenetic analysis of extreme morphologies: deep-sea holasteroid echinoids. Journal of Natural History 30, 913-953.

A. Agassiz 1904. Reports on an exploration off the west coasts of Mexico, central and south America and off the Galapagos Islands. Memoirs of the Museum of Comparative Zoology at Harvard University 31.