926 THE VOYAGE OF H.M.S. CHALLENGER. 



fenestrated triangular lattice wing. Therefore the skeleton becomes very similar to Clathrocorys 

 (PL 64, figs. 8-10), but wants the regular central cephalis. 



Dimensions. Length of the apical spine 0'2, of the basal spines 0'14. 



Habitat. Central Pacific, Station 271, depth 2425 fathoms. 



Genus 397. Periplecta, 1 Haeckel, 1881, Prodromus, p. 424. 



Definition P lectanida 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. (PI. 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 O'l7. 



Habitat. Central Pacific, Station 274, surface. 



1 Periphcta = Surrounded by a hunting net ; {/, a^tx.Tti. 



