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



Genus 399. Plectanium? Haeckel, 1881, Prodromus, p. 424. 



Definition. P 1 e c t a n i d a with six radial spines, arising in two opposite divergent 

 groups from the two poles of a common central rod. 



The genus Plectanium has been derived from Plagonium by concrescence of the 

 meeting branches, and bears therefore to it the same relation that the preceding 

 Hexaplecta exhibits to Hexaplagia. Whilst in these two latter genera the six spines 

 arise from a common central point, they arise here in two divergent groups from the 

 two poles of a horizontal common middle rod, similar to the spicula of many Beloidea. 



1. Plectanium trigeminum, n. sp. (PL 91, fig- H)- 



Spines straight and stout, six to eight times as long as the common middle rod, three-sided 

 prismatic ; in the distal half thickened, with three divergent terminal thorns ; in the basal half 

 with three to four verticils of thin lateral branches, which are again ramified, and by their united 

 threads produce an irregular loose framework. 



Dimensions. Length of the spines 0'25, basal breadth 0'005, terminal breadth 0'02; length of 

 the middle rod 0'03. 



Habitat. North Pacific, Station 244, surface. 



2. Plectanium ovodimare, n. sp. 



Spines straight and stout, three-sided prismatic, about four times as long as the common middle 

 rod, gradually thinned towards the distal end ; in the basal half with two to three verticils of 

 forked branches, which are dichotomously ramified, and by their united threads form a loose ovate 

 framework. 



Dimensions. Length of the spines 017, of the middle rod 0'04. 



Habitat. North Pacific, Station 236, surface. 



3. Plectanium sphcerozoum, n. sp. 



Spines straight, cylindrical, ten to twelve times as long as the common middle rod, with six to 

 eight verticils of thorny branches, tapering towards the distal end; the branches of the verticils 

 are simple in the distal half, again ramified in the basal half, and here connected together by 

 irregularly branched threads forming a loose framework. (Resembles one spiculum of SpJuxrozoum 

 verticillatum, PI. 4, fig. 7, but is more richly branched, with united ramules.) 



Dimensions. Length of the spines - 2, of the middle rod OD1. 



Habitat. Central Pacific, Station 266, surface. 



1 Plectanium = Small wickerwork or network ; 



