A seventh species of Plexechinus, Plexechinus sulcatus sp. nov., is described from material collected at 585 tn from an R/V "Marion Dufresne" station north-west of the Kerguelen Islands. It differs most markedly from its congeners in possessing: a distinctive aboral sulcus in the anterior ambulacrum, which is unique in the Plexechinidae. There is strong phylogenetic evidence that P. sulcatus is the sister group to a clade containing P. cinctus and P. hirsutus. The implications of this placement are discussed in the light of previous work on the evolutionary biology of holasteroids.
