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



3. Tetraplagia dbietina, Haeckel. 



Plagiacantha dbietina, var. quadrispina, Kichard Hertwig, 1879, Organismus d. Eadiol., p. 73. 



Spines straight, three-sided prismatic, verticillate, with six to eight verticils of three simple 

 straight branches ; the branches of each edge are parallel, tapering towards the distal end. 

 R Hertwig regards this species only as a four-spined variety of his three-spined Plagiacantha 

 abietina ; but a specimen, observed by me in Corfu, exhibited all the characters of Tetraplagia. 



Dimensions. Length of the spines 0'2, of the basal branches 0'07. 



Habitat. Mediterranean (Messina, Corfu), surface. 



Genus 387. Plagoniscus, 1 n. gen. 



Definition. P lagonida 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 Plagoniscus agrees with the preceding Tetraplagia (its probable 

 ancestral form) in the possession of four radial spines, diverging from one common 

 central point. But whilst in this latter all four spines are equal, corresponding 

 exactly to the four axes of a tetrahedron, here in Playiocarpa an important .difference 

 exists between one vertical or apical spine and three other divergent basal spines ; 

 these latter corresponding probably to the three " feet," the former to the single " apical 

 horn " of the majority of NASSELLARIA. Perhaps we find here one of the oldest 

 and simplest types of their " triradial or cortinar structure " (compare above, p. 902). 



1. Plagoniscus tripodiscus, n. sp. (PL 91, fig. 4). 



Spines three-sided prismatic, thorny. Apical spine nearly straight, verticillate, with four to 

 five verticils of three thorny branches, tapering towards the apex. Three basal spines somewhat 

 shorter, curved, with three thorny edges. 



Dimensions. Length of the apical spine 0'2, of the basal spines 015. 



Habitat. Central Pacific, Station 263, surface. 



2. Plagoniscus euscenium, n. sp. 



Spines three-sided prismatic, thorny, with dentate edges, and three to six verticils of three 

 short branches. Apical spine straight, with six verticils, nearly twice as long as the three curved 

 basal spines, each of which bears three verticils ; the basal verticils larger and ramified. Eesembles 

 somewhat Euscenium eucolpium, PI. 53, fig. 12, but has no latticed shell. 



Dimensions. Length of the apical spine 0'3, of the basal spines 0'16. 



Habitat. North Pacific, Station 247, surface. 



1 Plagoniscus Side windlass ; 



