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



the radius. (Similar to Hexastylus phcenaxonius, PI. 21, fig. 3, but differing in the unequal length 

 of the spines.) 



Dimensions. Diameter of the shell 015, pores 012, bars 0'004 ; length of the two major 

 spines 0'2, of the four minor 0'07. 



Habitat. Central Pacific, Station 265, depth 2900 fathoms. 



2. Hexastylarium quadratum, n. sp. 



Shell very delicate, with smooth surface, and irregular, polygonal pores, separated by very 

 thin bars. The form of the shell is not, as commonly, a sphere, but a geometrical square octahedron, 

 one axis (with two opposite major spines) being nearly twice as long as the other two dimensive 

 axes ; four spines, opposite by pairs in the latter, are only half as long. The eight sides of the 

 octahedral shell are even, equilateral-triangular. Spines angular, thin. 



Dimensions. Diameter of the shell in the major axis 018, in the minor 01 ; length of the 

 major spines 0'24, minor 012. 



Habitat. South Pacific, Station 295, depth 1500 fathoms. 



3. Hexastylarium elongatum, n. sp. 



Shell thick walled, with spiny surface, and with irregular, roundish pores, two to four times 

 as broad as the bars ; eight to ten on the radius. Two opposite major spines, three to four times 

 as long as the shell diameter, whilst the four other spines are very short, scarcely as long as the 

 radius. All six spines at the base three-sided pyramidal, the two longer being cylindrical. 



Dimensions. Diameter of the shell 01 ; length of the major spines 0'3 to 0'4, of the minor 

 0'04, basal breadth 0'02. 



Habitat. North Pacific, Station 244, surface. 



Genus 74. Hexastylidium, 1 Haeckel, 1881, Prodromus, p. 450. 



Definition.- C ubosphserida with one simple lattice-sphere and six simple spines 

 in pairs different ; the two opposite spines of each pair equal, the three pairs unequal. 



The genus Hexastylidium differs from its probable ancestral form, Hexastylus, by 

 the unequal growth of the six simple spines ; the two spines of each pair reaching the same 

 dimensions, whilst the three pairs are different. They correspond therefore to the three 

 axes of a rhombic crystal. 



1. Hexastylidium rhomboides, n. sp. 



Shell thin walled, smooth, with irregular, polygonal pores and very thin bars. Its form is not, 

 as commonly, a sphere, but a rhombic octahedron. The radial proportion of the three unequal 

 1 H cxastylidium = Shell with six styles ; derivation from Hexastylus, 



