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



Radial spines thirty to forty, prismatic, with three smooth, spirally contorted edges, without 

 lateral branches between the two shells ; their free outer part half as long as the shell radius. 



Dimensions. — Diameter of the spongy cortical shell 02, of its inner cavity 012, of the 

 medullary shell 0'04. 



Habitat. — Central Pacific, Station 265, surface. 



3. Rhizoplegma trigonacantha, n. sp. 



Spongy cortical shell with a rather loose framework and coarse bars, with nearly smooth surface 

 (without by-spines). Its inner cavity twice as broad as the medullary shell, which exhibits irregular, 

 roundish pores. Eadial spines forty to sixty, prismatic, with three smooth straight edges, without 

 free branches between the two shells ; their outer pyramidal part only one-third of the shell radius. 

 (Very similar to the common Rhizosphcera trigonacantha, but with simple medullary shell.) 



Dimensions. — Diameter of the spongy shell 0'2, of its inner ca\'ity 016, of the medullary shell 

 0-08. 



Habitat. — North Atlantic, Station 353, surface. 



Subgenus 2. Rhizoplegmidium, Haeckel. 



Definition. — Between the two shells free lateral branches arise from the three edges 

 of the radial spines. 



4. Rhizoplegma radicatum, u. sp. (PI. 15, figs. 9, 9a). 



Spongy cortical shell with a very loose framework ; on the surface are innumerable thin, forked, 

 or repeatedly dichotomous by-spines. Its inner cavaty three tunes as broad as the medullary shell, 

 which exhibits regular, hexagonal meshes (fig. 9a). Eadial spines twelve, prismatic, with straight 

 dentated edges, their outer pointed part as long as the shell radius. Each spine between the two 

 shells has a verticil of three forked lateral branches (fig. 9a). The central capsule completely 

 distends the medullary shell, and forces out through each mesh a hernia-shaped process (fig. 9). 



Dimensions. — Diameter of the spongy shell 0-5, of its mner cavity 0-25, of the medullary shell 

 0-08. 



Habitat. — Central Pacific, Station 271, surface. 



5. Rhizoplegma lychnosphoBra, n. sp. (PL 11, fig. 5). 



Lychnosphcera rhizoplegma, Haeckel, 1879, Atlas (pi. xi. fig. 5). 



Spongy cortical shell with a very loose framework, composed of long thin beams as in 

 Lychnosphcera regina (PI. 11, figs. 1^)- Surface covered with short bristles. Its inner cavity 

 six times as broad as the medullary shell, which exhibits regular, hexagonal meshes. Eadial 

 spines twelve, prismatic, with three smooth edges ; their outer pointed part half as long as the 

 shell radius. Each .spine has three verticils of three forked branches ; the first verticil is free 



