REPORT ON THE RADIOLARIA. XXIU 



equal radial spines. In the great majority of Acantharia, however (all A c a n t h o n i d a 

 and Acanthophracta), twenty radial spines are present, regularly distributed, 

 according to Miiller's icosacanthan law, in five parallel circles, each containing four 

 crossed spines (p. 717). Usually the twenty spines are equal, and the ground-form is 

 the quadratic octahedron, or a regular double pyramid with sixteen sides. But in some 

 groups (the Amphilonchida and Prunophracta) two opposite equatorial spines are much 

 more strongly developed than the other eighteen, and therefore the hydrotomical axis in 

 the equatorial plane is larger than the geotomical axis (p. 719) ; the isopolar stauraxonian 

 form passes over into the allopolar, and the ground-form becomes the rhombic octahedron 

 or the amphithect double pyramid (compare §§33 and 34, and p. 720). The centroplanar 

 ground-form is entirely wanting in the Acantharia. 



49. The Ground-Forms of the Nassellaria. — The Nassellaria aU possess monostatic 

 ground-forms, inasmuch as by the very structure of their monopylean central capsule a 

 vertical main axis is necessitated, whose basal pole occupies the porochora. The same 

 arrangement is also for the most part clearly recognisable in the corresponding structure 

 of the skeleton, which is generally either centraxon or centroplanar. Among their 

 manifold skeletal forms difierent larger groups of ground-forms may be recognised 

 according as the vertical allopolar main axis is crossed by differentiated transverse axes 

 or not (Stauraxonia or Monaxonia) ; the former are either triradial or multiradial. The 

 triradial, with three lateral or terminal radial apophyses, constitute the greater part of the 

 Nassellaria, and have probably been derived originally from the triradial Plectoidea 

 [Triplagia, Triplecta) ; a more careful examination, however (especially mth reference 

 to the structure of the cortinar septum), reveals the fact that the ground-form is not 

 strictly regularly pyramidal (with three equal radii), but amphipleural (with two paired 

 ventral and one unpaired dorsal radius), and that it usually passes over into a distinctly 

 zygopleural form. The same holds true of the multiradial Nassellaria, where for the 

 most part three interradial or six adradial (sometimes more) apophyses are intercalated 

 between the three primary perradial ones ; sometimes here also the ground-form is a 

 quite regular hexagonal or nonagonal pyramid, but usually it is more or less amphithect 

 or amphipleural. Among the eradial Nassellaria, which have no radial a23ophyses, the 

 ground-form is sometimes allopolar monaxon (conical, ovoid, hemispherical, &c.), some- 

 times amphithect pyramidal (even in the simplest Stephanida, Archichvus, &c.), or 

 sometimes distinctly zygopleural or bilateral (many Plectellaria). 



50. The Ground-Forms of the Phceodaria. — The Ph^odaria agree with the Nas- 

 SELARiA in the possession of a primitively centraxon ground-form, and like them are 

 monostatic, since a vertical main axis whose basal pole ^jasses through the astropyle is 

 present, owing to the' characteristic structure of their cannopylean central capsule. In 



