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



ring is the shorter axis of the frontal and equatorial ring, and about half as long as their longer 

 axis. All three rings smooth, without spines. 



Dimensions. — Height of the frontal ring 0'08, breadth 012. 



Habitat. — Central Pacific, Station 268, depth 2900 fathoms. 



3. Trissocircus octostoma (PI. 93, fig. 11). 



Sagittal ring circular, smaller than the other two rings, which are both equal, elliptical, distinctly 

 constricted at the poles of the principal and transverse axes. The axis of the circular sagittal 

 ring is the shorter axis of the frontal and equatorial ring, about one-third shorter than their longer 

 axis. All three rings armed with short, irregularly branched spiaes. 



Dimensions. — Height of the frontal ring Ol, breadth 0'12. 



Habitat. — Indian Ocean, Ceylon (Belligemma), Haeckel, surface. 



Subgenus 2. Tricirconium, Haeckel. 

 Definition. — All three rings equal, circular, 



4. Trissocircus octahedrus, n. sp. 



All three rings equal, circular, smooth, their rods cylindrical (in the transverse section circular). 

 From each pole of the three equal axes arises a short conical spine ; these six spines correspond to 

 the six corners of a regular octahedron. 



Dimensions. — Diameter of all three rings 0'08, thickness 0'006. 



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



5. Trissocircus globus, n. sp. (PL 82, fig. 13). 



All three rings equal, circular, thorny ; their rods prismatic (in the transverse section triangular), 

 with three sharp dentated edges and scattered branched thorns. From each pole of the three equal 

 axes (which .correspond to the three axes of a regular octahedron) arises a bunch of larger spines. 



Dimensions. — Diameter of all three rings 0'12, thickness O'Ol. 



Habitat. — Central Pacific, Station 268, depth 2900 fathoms. 



Genus 424. Trissocyclus,^ Haeckel, 1881, Prodromus, p. 446. 



Definition. — Cor on i da v.itli eight large, partly fenestrated gates of equal size. 

 Skeleton composed of three complete latticed rings, perpendicular to one another. 



The genus Trissocyclus differs from its nearly allied ancestral form Trissocircus only 

 in the development of small anastomosing branches along the edges of the rings. The 

 incomplete lattice-work, produced by these anastomoses, partly protects the eight large 

 gates. The three rings are either of equal or of different sizes. 



1 Trissocyclus = Composed of three crossed rings ; r^icuo;, x.vx.'ho;. 



