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



Diameter of the outer shell five times as large as that of the inner. Six radial spines very long, 

 arising from the six corners of the inner and piercing the spongy mass of the outer shell, consider- 

 ably exceeding it at the free distal end, three-sided prismatic, with elegantly denticulate edges. 



Dimensions. — Diameter of the outer shell 0'2, inner 0'04 ; total length of the spines 0"3 or more, 

 breadth 0-02. 



Habitat. — Central Pacific, Station 266, depth 2750 fathoms. 



Germs 87. Hexadoridium,^ Haeckel, 1881, Prodromus, p. 455. 



Dejinition. — C u b o s p h 83 r i d a with spongy splierical shell and two concentric 

 latticed medullary shells in its centre, having six simple spines of equal size. 



The genus Hexadoridium differs from Hexadoras in the duplication of the 

 medullary shell, and exhibits therefore the same relation to it that Spongodictyon 

 bears to Spongoi^legma. In the only known species the six spines are opposite, 

 arranged quite regularly in pairs in the three dimensive axes, and consequently 

 represent the three axes of a regular crystal or cube. 



1. Hexadoridium streptacanthum, n. sp. (PL 25, figs. 1, la). 



Both medullary shells spherical, with small regular, circular pores, twice as broad as the bars ; 

 outer twice as broad as the inner. Spongy cortical shell enclosing it with dense framework, five 

 times as broad as the outer medullary shell, regular octahedral. Six radial spines, arising from 

 the latter, are thinned at the inner end, three to five times as long as the diameter of the 

 cortical shell, and nearly as broad as the inner medullary shell, with three dentated and spirally 

 contorted edges. (Very similar to the common Spongospliwra streptacantha, with irregular and variable 

 number and disposition of spines ; possibly its ancestral form ?). 



Dimensions. — Diameter of the cortical shell 0'2, of the outer medullary shell 0'04, inner 0'02 ; 

 length of the spines 1 mm. and more, breadth 0'02. 



Habitat. — Central Pacific, Station 271, depth 2425 fathoms. 



Family X. Astrosph^rida (Pis. 11, 18-20, 26-30). 



Astrospharida, Haeckel, 1881, Prodromus, p. 449. 



Definition — S p h se r o i d e a with numerous (eight to twelve or more, commonly 

 between twenty and sixty) radial spines on the surface of the spherical shell ; living 

 soUtary (not associated in colonies). 



The family Astrosphserida, the largest and most varied of all S p h se r o i d e a, 

 is distinguished from the other members of this group by the possession of numerous 



' Hexadoridium = Shell with six small spears ; derivation from Hexadoras. 



