4. _Hexaplagia australis_, n. sp. Spines unequal, at unequal intervals, not opposite in pairs, cylindrical, irregularly branched and curved. In this species a definite arrangement of the six different spines could not be detected. _Dimensions._--Length of the spines 0.2 to 0.3, of their largest branches 0.1. _Habitat._--South of Australia, Station 160, surface. Genus 391. _Plagonium_,[10] Haeckel, 1881, Prodromus, p. 423. _Definition._--#Plagonida# with six radial spines, arising in two opposite divergent groups from the two poles of a common central rod. The genus _Plagonium_ differs from the preceding closely allied _Hexaplagia_ in the remarkable peculiarity, that the six radial spines do not arise from one common central point, but from the two poles of a common horizontal middle rod; three divergent spines on each pole. The skeleton of _Plagonium_ exhibits therefore the same remarkable form which is found in the isolated spicula of numerous #Beloidea# (_e.g._, _Lampoxanthium punctatum_, _Sphærozoum punctatum_), and bears the same relation to _Hexaplagia_ that _Plagonidium_ does to _Tetraplagia_. 1. _Plagonium sphærozoum_, n. sp. (Pl. 91, fig. 6). Spines straight and stout, three-sided prismatic, about twice as long as the common middle rod, irregularly branched or nearly verticillate, with short thorny branches. Similar to the single spicula of some species of _Sphærozoum_ and _Lampoxanthium_. _Dimensions._--Length of the spines 0.12, of the middle rod 0.06. _Habitat._--Equatorial Atlantic, Station 347, surface. {917}2. _Plagonium lampoxanthium_, n. sp. Spines irregularly curved, slender, cylindrical, six to eight times as long as the common middle rod, in the proximal half smooth, in the distal half covered with short thorns. (Similar to an isolated spiculum of _Lampoxanthium punctatum_ or of _Sphærozoum variabile_, Pl. 4, fig. 5.) _Dimensions._--Length of the spines 0.18, of the middle rod 0.03. _Habitat._--North Pacific, Station 240, surface. 3. _Plagonium arborescens_, n. sp. Spines irregularly curved and branched, slender, cylindrical, thorny, twelve to sixteen times as long as the middle rod; the branches are large, arborescent, their ramules again ramified and very thorny. _Dimensions._--Length of the spines 0.2 to 0.25, of the middle rod 0.02. _Habitat._--Indian Ocean, Madagascar (Rabbe), surface. 4. _Plagonium trigeminum_, n. sp. Spines straight, slender, cylindrical, four to six times as long as the middle rod, in the basal half smooth, in the distal half arborescent, with three to four irregular verticils of ramified branches. (Similar to a single spiculum of _Sphærozoum verticillatum_, Pl. 4, fig. 7.) _Dimensions._--Length of the spines 0.15 to 0.2, of the middle rod 0.05. _Habitat._--Central Pacific, Station 265, depth 2900 fathoms. 5. _Plagonium distriactis_, n. sp.