39. _Synopsis of the Geometrical Ground-Forms:--_
Principal Groups of Ground-Forms. Subsidiary Groups of Ground-Forms. Geometrical Type. Examples.
I. CENTROSTIGMA. The geometrical centre of the body is a point. Main axis wanting. | +-I. HOMAXONIA. | All axes equal. | | | +-1. _Sphere_, | | Central capsule of the #Sphæroidea# and of many ACANTHARIA. | +-II. POLYAXONIA. Endospherical polyhedra. All the angles of the body lie on the surface of a sphere. Numerous isopolar axes. | +-2. _Endospherical polyhedron_, | | Lattice-spheres of the #Sphæroidea#, #Sphærophracta#, and | #Phæosphæria#. | +-3. _Icosahedron_, | | _Circogonia_. | +-4. _Dodecahedron_, | | _Circorrhegma_. | +-5. _Octahedron_, | | Cubosphærida, _Circoporus_. | +-6. _Cube_, | | _Centrocubus_, _Lithocubus_, &c. | +-7. _Tetrahedron_,
II. CENTRAXONIA. { xix} The geometrical centre of the body is a straight line (the vertical main axis).
Constant transverse axes (perpendicular to the main axis) are wanting in the Monaxonia (which have circular transverse sections); on the contrary they are differentiated in the Stauraxonia (which have polygonal transverse sections). | +-III. MONAXONIA. | Uniaxial ground-forms or centraxonia without transverse axes. The | transverse planes (perpendicular to the main axis) are circles. | | | +-8. _Monaxonia isopola._ | | (Spheroids and ellipsoids; both poles of the main axis similar.) | | | | Central capsule and lattice-shell of of many #Discoidea# (lenses) and | | #Prunoidea# (ellipsoids), Belonaspida, &c. | | | +-9. _Monaxonia allopola._ | (Cone, ovoid and hemisphere; the two poles of the axis dissimilar.) | | Central capsule and lattice-shell of many NASSELLARIA, especially | the #Cyrtoidea# eradiata (Cyrtocalpida, &c.). | +-IV. STAURAXONIA. Pyramidal ground-forms or centraxonia with transverse axes. The transverse planes (perpendicular to the main axis) are either regular or amphithect polygons. | +-10. _Dipyramides regulares._ | (Quadratic octahedron, or quadrilonchial forms and regular double | pyramids.) | | ACANTHARIA with twenty radial spines, the four equatorial being | equal. Multiradial #Discoidea# and Staurosphærida. | +-11. _Dipyramides amphithectæ._ | (Rhombic octahedron, lentellipsoid, and amphithect double pyramids.) | | ACANTHARIA with twenty radial spines, whose four equatorial spines | are unequal but paired. Many #Larcoidea#. | +-12. _Pyramides regulares._ | (Regular pyramids.) | | Many NASSELLARIA (triradial and multiradial). Medusettida and | Tuscarorida. | +-13. _Pyramides amphithectæ._ (Rhombic pyramids.)