REPORT ON THE RADIOLARIA. 911 



i 



6. Plagiacantha elatine, n. sp. 



Spines straight, three-sided prismatic, with prominent wing-like edges and ten to twelve verticils 

 of three divergent branches, decreasing in size towards the distal end ; the branches of the three or 

 four basal verticils are again ramified and much larger than the simple branches of the distal 

 verticils. 



Dimensions. Length of the spines O36, of the basal branches 014. 



Habitat. Central Pacific, Station 268, depth 2900 fathoms. 



Subfamily 2. TETRAPLAGIDA, Haeckel, 1881, Prodromus, p. 424. 

 Definition. P lagonida with four radial spines. 



Genus 386. Tetraplagia, 1 Haeckel, 1881, Prodromus, p. 424. 



Definition. P lagonida with four equal radial spines, arising from one common 

 central point, and corresponding to the four axes of a tetrahedron. 



The genus Tetraplagia is one of the most important Plectoidea, and possibly 

 the common ancestral form of many NASSELLAEIA (compare above, p. 901). The 

 skeleton is composed of four radial rods, diverging from one common centre in different 

 directions, and corresponding to the four axes, which extend from the central point of a 

 tetrahedron to the central points of its four faces. The whole skeleton of Tetraplagia 

 exhibits therefore the same form, which is observed in the single quadriradiate spicula 

 of some Beloidea (in some species of Lampoxanthium, Spharozoum, &c.). 



1. Tetraplagia geometrica, n. sp. 



Spines straight, cylindrical, perfectly equal, corresponding in regular disposition exactly to the 

 four geometrical axes of a regular tetrahedron ; in the middle arise from each spine two opposite 

 equal lateral branches of half the length of the spine. 



Dimensions. Length of the spines 012, of the branches 0'06. 



Habitat. Central Pacific, Station 266, surface. 



2. Tetraplagia phcenaxonia, n. sp. (PL 91, fig. 3). 



Spines straight or slightly curved, three-sided prismatic, with irregular short branches arising 

 from the three edges ; the branches are thorny, tapering towards the apex. 

 Dimensions. Length of the spines 015, of the basal branches 0'03. 

 Habitat. Tropical Atlantic, Station 332, surface. 



1 Tetraplagia = Four-sided ; re-rj cnrAy/oj. 



