Genus 398. _Hexaplecta_,[17] Haeckel, 1881, Prodromus, p. 425. _Definition._--#Plectanida# with six radial spines, arising from one common central point. The genus _Hexaplecta_ has probably been derived from _Hexaplagia_ by concrescence of the meeting branches of the six spines, and therefore has to it the same relation that _Plectophora_ bears to _Plagiacantha_. The two species here described represent perhaps two different genera, since in the first all six spines are equal, opposed in pairs, in the second different in pairs (three upper smaller and three lower larger). 1. _Hexaplecta triaxonia_, n. sp. Spines equal, opposite in three crossed pairs, straight, three-sided prismatic, thorny, in the distal half with three leaf-shaped, dentate edges, in the basal half with three verticils of slender branches, which are connected by parallel threads, and so form an arachnoidal network with rectangular or rhomboidal meshes. _Dimensions._--Length of the spines 0.22, of their basal branches 0.06. _Habitat._--South Pacific, Station 285, depth 2375 fathoms. 2. _Hexaplecta tricladonia_, n. sp. Spines unequal, three larger spines (in the lower half of the body) being opposite to three smaller spines (in the upper half). The latter are nearly horizontally expanded, each bearing three straight divergent branches. The larger are stronger, twice as long and armed with three verticils, each of three branches. All six spines are slender, three-sided prismatic. Their branches are very thin, thread-shaped, and form by their union a loose network with irregular polygonal meshes. _Dimensions._--Length of the larger spines 0.25, of the smaller 0.12. _Habitat._--South Pacific, Station 291, surface. {928}Genus 399. _Plectanium_,[18] Haeckel, 1881, Prodromus, p. 424. _Definition._--#Plectanida# with six radial spines, arising in two opposite divergent groups from the two poles of a common central rod. The genus _Plectanium_ has been derived from _Plagonium_ by concrescence of the meeting branches, and bears therefore to it the same relation that the preceding _Hexaplecta_ exhibits to _Hexaplagia_. Whilst in these two latter genera the six spines arise from a common central point, they arise here in two divergent groups from the two poles of a horizontal common middle rod, similar to the spicula of many #Beloidea#. 1. _Plectanium trigeminum_, n. sp. (Pl. 91, fig. 11). Spines straight and stout, six to eight times as long as the common middle rod, three-sided prismatic; in the distal half thickened, with three divergent terminal thorns; in the basal half with three to four verticils of thin lateral branches, which are again ramified, and by their united threads produce an irregular loose framework. _Dimensions._--Length of the spines 0.25, basal breadth 0.005, terminal breadth 0.02; length of the middle rod 0.03. _Habitat._--North Pacific, Station 244, surface. 2. _Plectanium ovodimare_, n. sp.