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



Genus 79. Ilexacontium, Haeckel/ 1881, Prodromus, p. 452. 



Definition. — Shell ^\^tll three concentric lattice-spheres and six simple spines of equal 

 size. 



The genus Hexacordium, the ancestral form of the Hexacontida, is proljably 

 derived from Hexalonche by duplication of the medullary shell. As in the latter, aU 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. Ilexacontium phcBnaxonium, n. sp. 



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

 bars ; twelve to sLxteen 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 Hcxastylus phcenaxonius and from Hexalonche i^^^cenaxonia, PL 21, 

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



Dimensions. — Diameter of the outer sphere 0'2, middle O'l, inner 0'05 ; cortical pores 0'U08, 

 bars 0-0012 ; length of the spines 01. 



Habitat. — Central Pacific, Station 270, surface. 



2. Ilexacontium axotrias, n. sp. (PL 24, fig. 3). 



Cortical shell thui walled, smooth ; its jsores regular circidar, five tu 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 circuLu- 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. 



Dime7isions.— Diameter of the outer sphere 0'2, middle 0-07, inner 0'025 ; cortical pores O'OIG, 

 bars 0-003 ; length of the spines 0-1 to 0-015, basal breadth 0-02. 



Habitat. — North Pacific, Station 253, surface. 



3. Ilexacontium hexactis, Haeckel. 



Adinomma hexactis, Stohr, 1880, Palaeontogr., vol. xxvi. p. 91, Taf. ii. fig. 7. 

 Cortical shell tliick walled, smooth, or a httle rough. Pores regular circular, of the same 

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



1 ii(!a;aco)i<io)t=ShelI witli six darts; sJa.iojTio:'. 



