The genus, not previously recorded from New Zealand or Australia, is characterized as follows: Disc large, the intermediate actinal plates in several series, of which the innermost continue to about the middle of the arm. Marginals of both series well developed. Furrow spinelets forming a comb. No pedicellariae. Madreporite covered by paxillae.

Plutonaster knoxi sp. nov. Plate 1, Fig. C, holotype.

Diagnosis: Each marginal plate of both series carries one prominent, robust spine, this spine being surrounded by small spiniform granules. Adambulacral armature comprising a furrow-comb of about 8 uniform spines, outside of which is a single large subambulacral spine. Actinal intermediate plates carrying 1–3 large spines, in addition to a general coating of small spinules.

Material Examined: About 30 specimens (some of them greatly damaged) from the following localities: 330 fathoms, Station 41, Chatham Islands 1954 Expedition; 61 fathoms, Station 2, Mernoo Bank. Chatham Islands Expedition.

Remarks: This notable addition to the New Zealand fauna is one of the interesting discoveries of the expedition in 1954 led by Mr. George Knox. The full description, with photographic half-tone illustrations, will appear in the official report of the Chatham Islands Expedition. As can be seen from the diagnosis, the species resembles the North Atlantic Plutonaster bifrons (Wyville Thomson) and, like it, is distinguished from other species of the genus by having only a single large spine on each marginal plate of both series. The adambulacral armature also resembles that of P. bifrons. The two species are distinguished, however, by the armature of the actinal intermediate plates. In P. bifrons the actinal intermediate plates carry in addition to minute spinules, a single large spine, whereas in P. knoxi there are from one to three large spines, these spines intergrading into the coating of spinules in the case of the distal intermediate plates

Holotype: In the Canterbury Museum, R 105 mm, r 33 mm.