The genus _Plagoniscus_ agrees with the preceding _Tetraplagia_ (its probable ancestral form) in the possession of four radial spines, diverging from one common central point. But whilst in this latter all four spines are equal, corresponding exactly to the four axes of a tetrahedron, here in _Plagiocarpa_ an important difference exists between one vertical or apical spine and three other divergent basal spines; these latter corresponding probably to the three "feet," the former to the single "apical horn" of the majority of NASSELLARIA. Perhaps we find here one of the oldest and simplest types of their "triradial or cortinar structure" (compare above, p. 902). 1. _Plagoniscus tripodiscus_, n. sp. (Pl. 91, fig. 4). Spines three-sided prismatic, thorny. Apical spine nearly straight, verticillate, with four to five verticils of three thorny branches, tapering towards the apex. Three basal spines somewhat shorter, curved, with three thorny edges. _Dimensions._--Length of the apical spine 0.2, of the basal spines 0.15. _Habitat._--Central Pacific, Station 263, surface. 2. _Plagoniscus euscenium_, n. sp. Spines three-sided prismatic, thorny, with dentate edges, and three to six verticils of three short branches. Apical spine straight, with six verticils, nearly twice as long as the three curved basal spines, each of which bears three verticils; the basal verticils larger and ramified. Resembles somewhat Euscenium eucolpium, Pl. 53, fig. 12, but has no latticed shell. _Dimensions._--Length of the apical spine 0.3, of the basal spines 0.16. _Habitat._--North Pacific, Station 247, surface, {913}3. _Plagoniscus cortinaris_, n. sp. Spines three-sided prismatic, straight, verticillate. Apical spine with twelve verticils, one and a half times as long as the three basal spines, each of which bears eight verticils of three branches. The branches are also prismatic, straight, on each edge parallel, tapering towards the apex, in the three basal spines forked, in the apical spine more branched. Similar to the cortinar skeleton of _Clathrocorys_ (Pl. 64, figs. 8-10). _Dimensions._--Length of the apical spine 0.24, of the basal spines 0.16. _Habitat._--South Atlantic, Station 338, surface. 4. _Plagoniscus nassellaris_, n. sp. Spines cylindrical, curved, irregularly branched. Apical spine half as long as, and less branched than the three basal spines, which are sigmoidal, nearly horizontally expanded in the proximal half, descending in the distal half. _Dimensions._--Length of the apical spine 0.1, of the three basal spines 0.18. _Habitat._--West Tropical Pacific, Station 224, surface. Genus 388. _Plagonidium_,[7] Haeckel, 1881, Prodromus, p. 424. _Definition._--#Plagonida# with four equal radial spines, arising in pairs from the two poles of a common central rod.