REPORT ON THE RADIOLARIA. 285 



here one main axis is constantly larger than all other axes (Monaxonia). Usually 

 both poles of this main axis are equal (Haplopola); but in some genera both poles 

 become different (Diplopola). 



In the S p h se r o i d e a all planes going through the centre of the shell are 

 circular, whereas in the Prunoidea only those planes are circular which are perpendicu- 

 lar to the main axis ; all other planes going through the centre are elliptical ; the largest 

 of these are the meridian planes, in which is situated the main axis. Commonly all 

 meridian planes are equal, as no transverse axes (or cross axes) are differentiated. 



In my Monograph (1862) only very few forms of Prunoidea, such as Didymocyrtis 

 and Sponyurus, are described, and the greater part of them are distributed under different 

 genera of S p h se r o i d e a, such as Haliomma and Actinomma. In my Prodromus 

 (1881) I separated them as the family Zygartida (p. 462). But it seems now much more 

 convenient to restrict this term to a particular family and to give a wider extension to 

 the whole suborder under the name Prunoidea (called after the characteristic 

 ellipsoidal form of a plum, or Primus, with its stone). 



The suborder Prunoidea comprises seven different families, of which the 

 Ellipsida constitutes the simplest and the probable common ancestral group. In all 

 Ellipsida the fenestrated shell is simple, and never composed of concentric or twin shells. 

 In their primitive genus Cenellipsis, the whole shell is geometrically nothing more 

 than a simple ellipsoid (PL 39, figs. 1, 2). By development of radial spines it passes 

 into Ellipsidium, by development of spongy framework into Spongellipsis. In the 

 greater part of this family large spines are developed on both poles of the main axis 

 of the shell, but sometimes instead of these solid spines, two opposite hollow fenes- 

 trated tubes are developed (Pipettella, PI. 39, fig. 6). 



The second family of P r \\ n o i d e a, the Druppulida, is much richer in different 

 forms than the simple Ellipsida. In this case the ellipsoidal shell is composed of two 

 or three (rarely more) concentric shells. Constantly one or two of these fenestrated 

 shells are enclosed in the central capsule, and may therefore be called " medullary 

 shells "; and one or two (rarely more) lie outside the central capsule, " cortical shells." 

 The inner medullary shells (one or two) are either spherical or ellipsoidal ; the 

 outer cortical shells (one or two, rarely more) are always ellipsoidal. All concen- 

 tric shells are connected by radial beams. In the simplest form of the subfamily, 

 Druppula (PI. 39, fig. 3), one medullary shell is connected with one cortical shell. 

 By duplication of the medullary shell arises Prunulum (PI. 39, fig. 4), and by 

 duplication or multiplication of the cortical shell Cromyodruppa (PI. 15, figs. 1-4) 

 is formed. In by far the greater portion of this subfamily large spines are developed 

 on both poles of the main axis of the cortical shell (Pis. 16, 17), but sometimes 

 also instead of these solid spines two opposite hollow fenestrated tubes are developed 

 (Pipetta, PI. 39, figs. 7, 8). 



