30. _The Regular Tetrahedral Ground-Form._--The ground-form whose geometrical type is the regular tetrahedron, bounded by four equilateral triangles, occurs less frequently in the Radiolaria than the other four regular polyhedra. Among the SPUMELLARIA it is found in the #Beloidea#, and especially in those members of the Thalassosphærida and Sphærozoida whose spicules bear four equal branches, diverging at equal angles from a common centre. Precisely the same structure is seen also among the NASSELLARIA in some #Plectoidea#, as in _Tetraplagia_ among the Plagonida, and _Tetraplecta_ among the Plectanida. The skeleton of both these genera consists of four equal rods, which radiate at equal angles from a common centre, just as do the axes of the regular tetrahedron. The tetrahedral form of these #Plectoidea# is the more important and interesting since on the one hand it is related to the similar spicular form of the #Beloidea#, and on the other perhaps furnishes the starting point from which _Cortina_ among the NASSELLARIA may be derived (_Plagoniscus_, _Plectaniscus_). (See Gener. Morphol., Bd. i. p. 415.) 31. _The Isopolar-Monaxon or Phacotypic Ground-Form._--The isopolar uniaxial or phacotypic ground-form is characterised by the possession of a vertical main axis with {xv}equal poles, whilst no transverse axes are differentiated. All horizontal planes which cut the axis at right angles are circles, and increase in size from the poles towards the equator. The most important ground-forms of this group are the _phacoid_ (the lens or oblate spheroid) and the _ellipsoid_ (or prolate spheroid). Phacoids (or geometrical lenses with blunt margins) are very often presented by the central capsules of the #Discoidea# and of many ACANTHARIA (Quadrilonchida and Hexalaspida), but the lattice-shells of many SPUMELLARIA and ACANTHARIA exhibit the same form, as also do a few PHÆODARIA (_e.g._, _Aulophacus_). True geometrical ellipsoids are seen in the central capsules of many #Prunoidea# among the SPUMELLARIA, and of many Amphilonchida and Belonaspida among the ACANTHARIA. Furthermore, the lattice shells of many species of these groups retain the same essential form, _e.g._, many Ellipsida, Druppulida, and Spongurida (Pls. 13-17, and 39), as well as most Belonaspida. (See Gener. Morphol., Bd. i. p. 422.)