200 THE VOYAGE OF H.M.S. CHALLENGES. 



2. Hexadendron bipinnatum, n. sp. (PI. 23, fig. 1). 



All three shells regular octahedral, with very delicate network of irregular polygonal meshes, 

 and very thin, thread-like bars between them ; their radial proportion = 1 : 2 - 5 : 7'5. Surface of 

 the cortical shell covered with numerous bristle-shaped by-spines, as long as the radius. Six main 

 spines very large, three-sided prismatic, with three rows of pinnate, lateral branches, on the three 

 wing-like, spirally twisted edges (similar to those of Hexancistra mirabilis, p. 189, PL 23, fig. 3). 



Dimensions. Diameter of the outer shell 012, middle O04, inner O'OIG. 



Habitat. Central Pacific, Station 272, surface. 



Genus 81. Hexacontarium, 1 n. gen. 



Definition. C ubosphaerida with three concentric lattice-spheres and six 

 simple spines of different size ; one opposite pair larger than the two others. 



The genus Hexacontarium exhibits to its ancestral form Hexacontium the same 

 relation that Hexaloncharium bears to Hexalonche ; the former is developed from the 

 latter by duplication of the medullary shell. As two opposite spines of one pair are 

 larger than the four others, they correspond to the three axes of a quadratic crystal. 



1. Hexacontarium dentatum, n. sp. 



Cortical shell with regular circular, hexagonally framed pores, twice as broad as the bars, with 

 smooth surface. Pvadial proportion of the three spheres = 1:2:5. Six radial spines three-sided 

 prismatic, half as broad as the inner medullary shell, with three dentated edges. Two opposite 

 major spines longer than the shell diameter ; four other minor spines only half as long as the 

 radius. (Similar to Hexacontium clavigerum, PI. 23, fig. 5, but distinct by the prolongation of the 

 spines of one axis.) 



Dimensions. Diameter of the outer shell 0'15, middle 0'06, inner 0'03 ; length of the major 

 spines 0'2, minor 0'04. 



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



2. Hexacontarium clavatum, n. sp. 



Cortical shell covered with short conical by-spines and irregular roundish pores, three to four 

 times as broad as the bars. Eadial proportion of the three spheres = 1:3:8. Six radial spines 

 three-sided prismatic, at the distal end club-shaped ; two major spines three times as long as the 

 four others, which are equal to the shell radius. 



Dimensions. Diameter of the outer shell 0'16, middle 0'06, inner 0'02. 



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



1 Hexacontarium = Shell with six darts ; derivation, from Hexacontion. 



