Four spines arising in two pairs from the poles of a common central rod; one apical spine different common spines, 397. _Periplecta_. ----------------------------------------------------------------- III. Subfamily Hexaplectida. Six radial spines. ----------------------------------------------------------------- Six spines arising from one common central point, 398. _Hexaplecta_. Six spines arising in two opposite groups from the poles of a common control rod, 399. _Plectanium_. ----------------------------------------------------------------- IV. Subfamily Polyplectida. Numerous radial spines. ----------------------------------------------------------------- Numerous (seven to nine or more) radial spines arising from a common centre (either a central point or a branched rod), 400. _Polyplecta_. Subfamily 1. TRIPLECTIDA, Haeckel, 1881, Prodromus, p. 424. _Definition_.--#Plectanida# with three radial spines. Genus 393. _Triplecta_,[12] Haeckel, 1881, Prodromus, p. 424. _Definition_.--#Plectanida# with three radial spines, arising from one common central point and lying in one horizontal plane. The genus _Triplecta_ is the simplest and most primitive of the Plectanida, and may therefore be regarded as the prototype of this family. The skeleton represents a triangular lattice-plate with three radial beams. On the central union of the latter rests the oral pole of the central capsule. Since the axis of the latter is vertical, the lattice-plate must be horizontal, serving for the expansion of the pseudopodia. _Triplecta_ has arisen from _Triplagia_ by union of the branches of its three radial spines. In the simplest case only three large meshes are formed, corresponding probably to the three cortinar meshes in the collar septum of many #Cyrtellaria#. 1. _Triplecta triangulum_, n. sp. Spines straight, equal, smooth, cylindrical, each in the basal half with one pair of divergent straight lateral branches. The opposed branches of every two neighbouring spines are united by a {922}thin convex bow. In this way arises a very simple, equilateral triangular skeleton, with three equal large meshes. _Dimensions._--Length of the spines 0.14, sides of the triangle 0.16. _Habitat._--Tropical Atlantic, Station 348, depth 2450 fathoms. 2. _Triplecta triactis_, n. sp. (Pl. 91, fig. 7). Spines straight, equal, three-sided prismatic, pinnate, each with three to four pairs of opposite slender pinnulæ or lateral branches; the pinnulæ of each spine are correspondingly parallel to the main rods of the other two spines. All pinnulæ branched and connected by a few slender bows, marking the sides of a regular triangle. _Dimensions_.--Length of the spines 0.15, sides of the triangle 0.17. _Habitat._--Central Pacific, Station 272, depth 2600 fathoms. Genus 394. _Plectophora_,[13] Haeckel, 1881, Prodromus, p. 424.