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



Genus 379. Hexacolpns,^ n. gen. 



Definition. — H e x a 1 a s ^i i tl a with six prominent radial spines (in the hydrotomical 

 plane), which are surrounded by prominent sheaths at the base ; the fourteen other 

 spines quite rudimentary, not prominent. 



The genus Hexacolpus differs from the preceding Hexonasjns in the development 

 of conical or cylindrical sheaths around the basal parts of the radial spines. It bears to 

 the latter the same relation as Hexaconus exhibits to Hexalaspis. 



Subgenus 1. Hexacolparium, Haeckel. 

 Definition. — Six hydrotomical spines all nearly of ecjual size. 



1. Hexacolpus nivalis, n. sp. (PI. 139, fig. 1). 



Hexalaspis nivalis, Haeckel, 1882, Manuscript et Atla?. 



All six hydrotomical spines of nearly equal size and equidistant, thin, lanceolate, compressed, 

 somewhat sliorter than the diameter of the hexagonal shell. Sheaths nearly prismatic, conical in the 

 distal third, each with twelve prominent parallel crests, which are separated by twelve deep 

 furrows ; their contracted distal opening or mouth denticulate. The sheaths are as long as the 

 radius of the shell, and envelop two-thirds of the spines. (EesemLles certain forms of snow- 

 crystals.) 



Dimensions. — Diameter of the shell 015, thickness of its wall 0'08 ; length of the spines Ol, of 

 the sheath 0-007 ; breadth of the latter 0-04. 



Habitat. — West Tropical Pacific, Station 225, surface. 



2. Hexacolpus conifer, n. sp. 



All .six hydrotomical .spines of ecpial size and equidistant, triangular, compressed, about as long 

 as the radius of the circular shell. Sheatlis conical, sulcate, half as broad on the serrate distal end 

 as on the base. The sheaths envelop the basal half of the spines. 



Dimension.^. — Diameter of the shell 0-18; length of the sjiines O'l, of the sheatlis 0-06 ; basal 

 breadth of the latter 0-07, distal breadth 0-04. 



Habitat. — North Pacific, Station 236, surface. 



Subgenus 2. Hexacolpidium, Haeckel. 



Definition. — Six hydrotomical spines of unequal size, two opposite (equatorial) much 

 larger than the four other (polar) spines. 



' Hexacolpi(s = Shell 'with six bosoms ; 6|«, xoXtos. 



