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



of the central cube arise eight primary radial spines, and often others from the 

 framework between them. 



The genus Centrocubus and the following closely allied Octodendroii may represent 

 a peculiar small group of Spongiommida, remarkable for the regular, cubical form 

 of the medullary shell, which is composed of twelve thin rods, corresponding to the 

 twelve edges of a mathematical cube ; from the eight corners invariably arise eight 

 primary radial spines, the branchers of which form the spongy cortical shell. 



1. Centrocubus octostylus, n. sp. 



Eadial spines eight, arising from the eight corners of the cubiform, regular, medullary 

 shell, gradually increasing in thickness towards the club-shaped distal end, which is five to 

 six times as broad as the central end. From the three denticulate edges of each spine arise six 

 to eight lateral branches, which ramify irregularly and form by their anastomosis the spongy 

 framework which is of nearly similar structure throughout, and with large loose meshes. The 

 free distal part of each spine is half as long as the enclosed part. 



DimensioTis. — Diameter of the sphere 06, of the central cube 0'02 ; length of the spines 015, 

 distal thickness 0"02. 



Habitat. — Central Pacific, Station 271, surface. 



2. Centrocxibus cladostylus, n. sp. (PI. 18, fig. 1). 



Eadial spines thirty-two, club-shaped, at the distal end eight to ten times as broad as at the 

 basal end. Eight primary spines arise from the eight corners of the regular, cubiform medullary 

 shell, and from these, in the form of lateral branches, twenty-four secondary spines arise with 

 concavely curved bases (three from the three denticulate edges of each spine, at nearly equal 

 distances from the centre). The free distal end of each of the thirty-two spines is of the same 

 shape, about half as long as the radius of the spongy sphere ; framework much looser in the 

 outer than in the inner part. 



Dimensions. — Diameter of the sphere 08, of the central cube 0-02 ; length of the spines 0-2, 

 distal thickness 0"02. 



Habitat. — North Pacific, Station 256, surface. 



3. Centrocubus polystylus, n. sp. 



Eadial spines sixty to eighty, club-shaped, four to six times as broad at the distal as at the 

 basal end. Eight primary spines arise from the eight corners of the regular, cubiform medullary 

 shell, the remainder either springing as lateral branches from the three denticulate corners of the 

 former, or arising within the spongy framework, which is much denser and darker in the central 

 than in the peripheral part. The free distal end of each spine is one-third as long as the radius. 



