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.