Nine radial spines of equal size, straight, cylindrical, lying nearly in a horizontal plane, or forming a very flat pyramid. Near the common central point every three spines arise from a short common rod, so that the centre of the skeleton is triradial. Each spine bears towards the apex two divergent straight lateral branches. This species may have been derived from _Plagiacantha arachnoides_ by shortening and reduction of the basal parts of the three original branched primary spines. _Dimensions._--Length of the nine spines 0.24, of the three basal rods 0.02. _Habitat._--North Atlantic, Færöe Channel (Gulf Stream), John Murray, surface. 4. _Polyplagia duodenaria_, n. sp. Twelve radial spines of equal size, arising from a common central point, and diverging in different directions. The twelve spines are very large, opposite in six pairs, cylindrical, longitudinally {919}striped (the expression of concentric lamellæ), and with spinulate surface, covered with innumerable small thorns. The basal quarter of each spine is straight and simple, the second quarter twice forked, and these four fork-branches are again in the outer half of the spine richly forked or ramified, with diverging, slightly curved thin branches; each of the twelve spines with about sixty to eighty terminal branches, the ends of which seem to fall into a spherical face. The position of this remarkable species in this family is doubtful. _Dimensions._--Length of the spines 0.12 to 0.16, of the simple basal part 0.04. _Habitat._--South Pacific (off Juan Fernandez), Station 299, surface. 5. _Polyplagia viminaria_, n. sp. Numerous (sixteen to twenty or more) radial spines of about equal size, arising from a common central point and diverging in different directions, richly and more or less irregularly branched. The ends of the numerous small branches seem to fall into a spherical face. The large spines of this species have the same form and structure as in the preceding, nearly allied species, but are more numerous and more irregularly branched and disposed. _Dimensions._--Length of the spines 0.2 to 0.25, of the simple basal part 0.05. _Habitat._--North Pacific, Station 241, surface. Family XLVII. #Plectanida#, Haeckel. _Plectanida_, Haeckel, 1881, Prodromus, p. 424. _Definition._--#Plectoidea# with a wattled skeleton, composed of the meeting and united branches of radial spines, which arise from a common central point or central rod, and protect the partly enclosed central capsule.