The Echinoid Directory

• Test small, ovate; thin-shelled. No anterior sulcus in type species.
• Apical disc with single anterior genital plate. Usually with four gonopores (may be three). In type species, plating highly anomalous in being highly disjunct (see Mooi & David 1996).
• Ambulacra all similar with minute pore-pairs only; all flush.
• Peristome subcircular and well removed from anterior margin.
• Plastron plating orthosternous; sternal plate enlarged and followed by a pair of symmetrical episternal plates.
• Plastron usually disjunct with labrum separated from sternal plate by single pair of ambulacral plates that meet at the midline.
• Scattered tubercles uniformly distributed over the entire aboral surface.
• Periproct supramarginal and slightly depressed.
• Subanal fasciole present, starting on Iamb plates 2a/3b.
• Globiferous pedicellariae with very foreshortened neck.

Recent; Pacific, North Atlantic, Pacific and Antarctic.

Plexechinus cinctus A. Agassiz, 1898, p.78, by original designation.

• P. cinctus A. Agassiz, 1898; Recent, East Pacific.
• P. aoteanus (McKnight 1974); Recent, New Zealand and Antarctic [includes P. parvus (Mirnov) and P. planus (Mirnov)].
• P. hirsutus Mortensen 1905; Recent, North Atlantic.
• P. spectabilis Mortensen, 1948; Recent, Sulu Sea, Philippines.
• P. sulcatus David & Mooi, 2000; Recent, Antarctic.

Holasteroida, Meridosternata, Urechinina, Plexechinidae.

May be a grade taxon.

Like Cystechinus A. Agassiz in having an apical disc with (usually) three gonopores and a single anterior genital plate. Differs in having the plastron disjunct and from most other urechinids in having a subanal heel. Only A. nordenskjoldi (Mortensen) has a similar shape. Mooi & David (1996) provide a detailed analysis of this genus.

Plexechinus was placed in its own family, Plexechinidae by Mooi & David (1996), citing two synapomorphies (the \'fused\' anterior genital plate and the position of the subanal fasciole crossing 5.a.5 and 5.b.6 aborally). Both characters are also found in Galeaster and are therefore probably primitive for pourtalesiids in general.

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.

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