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



which are three-sided prismatic ; their outer free distal end only as long as the diameter of the 

 inner medullary shell. 



Dimensions. Diameter of the spongy shell 0'25, of its inner cavity 0'2, outer medullary shell 

 01, inner 0'05. 



Habitat. Cosmopolitan ; Mediterranean, Atlantic, Indian, Pacific, surface from many Stations. 



2. Rhizosphcera serrata, n. sp. (PI. 18, figs. 57). 



Central cavity of the spongy shell five times as broad as the diameter of the outer medullary 

 shell. Bars of all three shells of the same breadth as the forty to sixty (or more) thin radial beams 

 between them. These are three-sided prismatic, with denticulate edges, scarcely half as broad as 

 their outer prolongations, which are half as long as the shell radius, and possess three spirally 

 contorted serrated edges. (The figured specimen is a young one ; in the older specimens the spongy 

 framework of the cortical shell is much more developed.) 



Dimensions. Diameter of the spongy shell 0'3, of its central cavity 0'22, outer medullary shell 

 0-06, inner 0'02. 



Habitat. Central Pacific, Stations 270 to 274, surface. 



3. Rhizosphcera leptomita, Haeckel. 



RMzosphcera leptomita, Haeckel, 1862, Monogr. d. Eadiol., p. 453, Taf. xxv. figs. 8-10. 



Central cavity of the spongy cortical shell twice as broad as the diameter of the outer medullary 

 shell ; bars of both very thin, only one-third as broad as the bars of the inner medullary shell. 

 Eadial spines thirty to fifty (or more), curved, three-sided prismatic ; inside the spongy shell as 

 thin as their bars, outside three times as broad. 



Dimensions. Diameter of the spongy shell 0'27, of its inner cavity 0'2, outer medullary shell 

 01, inner 0'05. 



Habitat. Mediterranean (Messina) ; Atlantic, Stations 348 to 354, surface. 



Suborder PEUNOIDEA, Haeckel, 1883 (Pis. 13-17, 39, 40). 



Definition. SPUMELLARIA with an ellipsoidal or cylindrical central capsule, prolonged 

 into one axis (sometimes articulate by annular transverse strictures) ; with an ellipsoidal or 

 cylindrical, fenestrated siliceous shell (often articulate by annular transverse strictures), 

 invariably prolonged into one axis. Fundamental form monaxon, usually with the 

 poles of the prolonged dimensive main axis equal. 



The suborder Prunoidea comprises those SPUMELLARIA in which the fenestrated 

 spherical shell appears prolonged into one axis. The geometric fundamental form of 

 the shell, which in the Sphseroidea was a sphere, in this case therefore becomes an 

 ellipsoid, and whilst in the former all axes originally have the same value (Homaxonia), 



