_Definition._--#Plagonida# with four equal radial spines, arising from one common central point, and corresponding to the four axes of a tetrahedron. The genus _Tetraplagia_ is one of the most important #Plectoidea#, and possibly the common ancestral form of many NASSELLARIA (compare above, p. 901). The skeleton is composed of four radial rods, diverging from one common centre in different directions, and corresponding to the four axes, which extend from the central point of a tetrahedron to the central points of its four faces. The whole skeleton of _Tetraplagia_ exhibits therefore the same form, which is observed in the single quadriradiate spicula of some #Beloidea# (in some species of _Lampoxanthium_, _Sphærozoum_, &c.). 1. _Tetraplagia geometrica_, n. sp. Spines straight, cylindrical, perfectly equal, corresponding in regular disposition exactly to the four geometrical axes of a regular tetrahedron; in the middle arise from each spine two opposite equal lateral branches of half the length of the spine. _Dimensions._--Length of the spines 0.12, of the branches 0.06. _Habitat._--Central Pacific, Station 266, surface. 2. _Tetraplagia phænaxonia_, n. sp. (Pl. 91, fig. 3). Spines straight or slightly curved, three-sided prismatic, with irregular short branches arising from the three edges; the branches are thorny, tapering towards the apex. _Dimensions._--Length of the spines 0.15, of the basal branches 0.03. _Habitat._--Tropical Atlantic, Station 332, surface. {912}3. _Tetraplagia abietina_, Haeckel. _Plagiacantha abietina_, var. _quadrispina_, Richard Hertwig, 1879, Organismus d. Radiol., p. 73. Spines straight, three-sided prismatic, verticillate, with six to eight verticils of three simple straight branches; the branches of each edge are parallel, tapering towards the distal end. R. Hertwig regards this species only as a four-spined variety of his three-spined _Plagiacantha abietina_; but a specimen, observed by me in Corfu, exhibited all the characters of _Tetraplagia_. _Dimensions._--Length of the spines 0.2, of the basal branches 0.07. _Habitat._--Mediterranean (Messina, Corfu), surface. Genus 387. _Plagoniscus_,[6] n. gen. _Definition._--#Plagonida# with four unequal radial spines, arising from one common central point; one vertical or apical spine opposed to three divergent or basal spines.