50. _The Ground-Forms of the Phæodaria._--The PHÆODARIA agree with the NASSELLARIA in the possession of a primitively centraxon ground-form, and like them are monostatic, since a vertical main axis whose basal pole passes through the astropyle is present, owing to the characteristic structure of their cannopylean central capsule. In {xxiv}the great majority of PHÆODARIA the spheroidal central capsule also possesses a pair of parapylæ near the opposite apical pole of the main axis (Tripylea), and these determine (as the right and left secondary openings) an isopolar frontal axis. Hence, strictly speaking, in most PHÆODARIA the central capsule has the geometrical ground-form of the amphithect pyramid (as in the Ctenophora), with an allopolar vertical main axis, and two unequal, but isopolar, horizontal transverse axes. In many PHÆODARIA the skeleton also has this amphithect pyramidal ground-form, _e.g._, the bivalved #Phæoconchia# and part of the #Phæogromia#. On the contrary, in the rest of the PHÆODARIA the skeleton exhibits very various geometrical ground-forms, independent of that of the central capsule. In the #Phæosphæria# it forms preferably spheres or endospherical polyhedra, as also in the Castanellida and Circoporida among the #Phæogromia#; among the Circoporida there are also seen with remarkable distinctness the regular polyhedra (especially the dodecahedron and icosahedron). Isopolar monaxonia are found among the Aulosphærida (_Aulatractus_) and Orosphærida; allopolar monaxonia among the Challengerida (_Lithogromia_). The Medusettida and Tuscarorida show various forms of regular pyramids (allopolar Stauraxonia); and finally, the Challengerida are for the most part centroplanar or bilateral. Thus the PHÆODARIA present a great wealth of different geometrical ground-forms in the development of their skeleton, not in that of their central capsule. CHAPTER II.--THE CENTRAL CAPSULE.