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



Genus 79. Hexacontium, Haeckel, 1 1881, Prodromus, p. 452. 



Definition. Shell with three concentric lattice-spheres and six simple spines of equal 

 size. 



The genus Hexacontium, the ancestral form of the Hexacontida, is probably 

 derived from Hexalonche by duplication of the medullary shell. As in the latter, all six 

 spines are of equal size, opposite in pairs in the three dimensive axes, and correspond 

 therefore to the three equal axes of a tesseral crystal. 



Subgenus 1. Hexacontanna, Haeckel. 



Definition. Pores of the cortical shell regular or subregular, of nearly equal size and 

 similar form ; surface smooth, without radial spines or papillae (other than the six main 

 spines). 



1. Hexacontium phasnaxonium, n. sp. 



Cortical shell thin walled, smooth ; its pores regular hexagonal, six to nine times as broad as the 

 bars ; twelve to sixteen on the radius. Eadial proportion of the three spheres =1:2:4. Both 

 medullary shells of the same structure as the cortical shell, only with smaller pores. The three 

 spheres connected by six thin radial beams, which are prolonged on the outside into six strong 

 triangular pyramidal spines, as long as the radius of the cortical shell, and, at the base, as broad as 

 one of its pores. (Differs from Hexastylus phcenaxonius and from Hexalonche phcenaxonia, PI. 21, 

 fig. 3, in the larger size and the triple shell.) 



Dimensions. Diameter of the outer sphere 0'2, middle 01, inner 0'05 ; cortical pores O'OOS, 

 bars 0'0012 ; length of the spines 01. 



Habitat. Central Pacific, Station 270, surface. 



2. Hexacontium axotrias, n. sp. (PI. 24, fig. 3). 



Cortical shell thin walled, smooth ; its pores regular circular, five to six times as broad as the 

 bars ; eleven to thirteen on the radius. Eadial proportion of the three spheres =1:3:8. 

 Outer medullary shell with hexagonal frames around the regular circular pores (five to six on the 

 radius). Inner medullary shell with simple small circular pores (three on the radius). The two 

 outer shells connected by six triangular prismatic beams, which are prolonged outside to the length 

 of the cortical radius or more. 



Dimensions. Diameter of the outer sphere - 2, middle 0'07, inner 0'025 ; cortical pores O'OIG, 

 bars 0-003 ; length of the spines 01 to 0'015, basal breadth 0"02. 



Habitat. North Pacific, Station 253, surface. 



3. Hexacontium hexactis, Haeckel. 



Aetinomma hexactis, Stb'hr, 1880, Palaeontogr., vol. xxvi. p. 91, Taf. ii. fig. 7. 



Cortical shell thick walled, smooth, or a little rough. Pores regular circular, of the same 

 breadth as the bars; five to seven on the radius. Eadial proportion of the three spheres 



1 Hexacontion = Shell with six darts; 



