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



Genus 379. Hexacolpus, 1 n. gen. 



Definition. Hexalaspida 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 Hexonaspis 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. 

 i 

 Definition. Six hydrotomical spines all nearly of equal size. 



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



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



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

 somewhat shorter 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. (Resembles certain forms of snow- 

 crystals.) 



Dimensions. Diameter of the shell 0'15, thickness of its wall 0'08 ; length of the spines 01, 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 equal size and equidistant, triangular, compressed, about as long 

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

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



Dimensions. Diameter of the shell O18 ; length of the spines (H, of the sheaths 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. 



s = Shell with six bosoms ; t , xo'Axof. 



