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



All pinnulse connected by a few slender bridges, thus producing a delicate network with irregular 

 rhomboidal meshes. 



Dimensions. Length of the spines 0'15, of the basal branches 0'05. 



Habitat. Central Pacific, Station 272, depth 2600 fathoms. 



2. Tetraplecta quadricornis, n. sp. 



Spines in the basal half straight, in the distal half slightly curved and irregularly branched, 

 with two to three pairs of unequal alternate lateral branches) the distal ends of which are 

 connected by a few slender bows, marking the six edges of an irregular tetrahedron. 



Dimensions. Length of the spines 0'22, of the basal branches 0'14. 



Habitat. Central Pacific, Station 266, depth 2750 fathoms. 



3. Tetraplecta pinigera, n. sp. (PI. 91, fig. 8). 



Spines three-sided prismatic, straight, or in the basal half slightly curved, verticillate ; each 

 spine with six to eight three-branched regular verticils, tapering gradually towards the distal end. 

 All branches of each spine lie parallel in three equidistant meridian planes, and are connected by 

 delicate parallel threads, perpendicular to the branches. Therefore the skeleton consists of four 

 pine-shaped trees and twelve delicate triangular wings with rectangular meshes. 



Dimensions. Length of the spines - 25 to 0'3, of the basal branches 01 to - 15. 



Habitat. Central Pacific, Station 271, surface. 



Genus 396. Plectaniscus, 1 nov. gen. 



Definition. P lectanida with four unequal radial spines, arising from one 

 common central point ; one vertical or apical spine opposed to three divergent or basal 

 spines. 



The genus Plectaniscus has probably been derived from Plagoniscus by reticular 

 union of the neighbouring branches of the four radial spines, and exhibits therefore to it 

 the same relation that Tetraplecta bears to Tetraplagia. Whilst in these two latter 

 genera the four spines are equal, in the two former genera there is an important differ- 

 ence between a vertical spine (or apical horn) and three divergent (commonly larger) 

 spines, corresponding to the three basal feet of the triradiate NASSELLARIA. Perhaps, 

 therefore, Plectaniscus is one of the remotest ancestors of the latter ; but differs in the 

 absence of a complete lattice-shell. 



1 Plectaniscus = Shell of wickerwork ; 



