_Definition._--#Plectanida# with four unequal radial spines, arising in pairs from the two poles of a common central rod; one ascending apical spine opposed to three descending basal spines. The genus _Periplecta_ has probably been derived from _Plagiocarpa_ (by concrescence of the meeting spine branches), and has the same important relation to a part of the triradial #Stephoidea# (_Cortina_, &c.) and #Cyrtoidea# (_Pteroscenium_, &c.). It differs from the latter only in the fact that the loose irregular framework connecting the bases of the four cortinar spines is not a regular lattice-shell. 1. _Periplecta cortina_, n. sp. (Pl. 91, fig. 10). Spines of very different size and shape, three-sided prismatic. The apical spine (or horn) is larger, nearly straight, and bears a verticil of three large divergent branches, which are again ramified. The three basal spines (or feet) are curved, pinnate, each with three to five pairs of opposite simple curved branches. A small common central rod separates the two united pectoral feet from the two other spines (the caudal foot and the apical horn). The three basal feet are connected by three convex bows composing a horizontal collar ring, and since the central rod is prolonged between the pectoral feet to the ring, a very remarkable collar septum is formed with four cortinar meshes. Above this septum an irregular spongy shell, including the central capsule, is formed by a delicate framework, interwoven between the apical horn and the three basal feet. _Dimensions._--Length of the apical spine 0.25 to 0.3, of the three basal spines 0.15 to 0.2. _Habitat._--Central Pacific, Station 271, surface. 2. _Periplecta pteroscenium_, n. sp. Spines of very different size and shape, three-sided prismatic, verticillate. The apical spine (or horn) with ten to twelve verticils, is one and a half times as long as the three basal spines (or feet), which bear six to eight verticils only; each verticil with three branches which by communicating ramules form a loose irregular wickerwork. Similar to _Pteroscenium pinnatum_, Pl. 53, figs. 14-16, but without a regular lattice-shell. A short basal central rod separates two equal (pectoral) spines from two unequal spines, the larger of the latter is the apical, the shorter the caudal spine. _Dimensions._--Length of the apical spine 0.26, of the three basal spines 0.17. _Habitat._--Central Pacific, Station 274, surface. {927}3. _Periplecta monocyrtis_, n. sp. Spines of nearly equal size and shape, cylindrical, irregularly branched. The apical spine is straight, more branched and nearly twice as long as the three basal spines. All four spines in the basal third connected by a loose spongy framework, approaching the form of some Monocyrtida. _Dimensions._--Length of the apical spine 0.24, of the basal spines 0.13. _Habitat._--Central Pacific, Station 274, surface. Subfamily 3. HEXAPLECTICA, Haeckel. _Definition._--#Plectanida# with six radial spines.