apical (or aboral). Of the three feet of the basal tripod the unpaired (caudal) one is directed dorsally and backwards, the two paired (pectoral) ones ventrally and forwards. The majority of the NASSELLARIA may be regarded as modifications of this original ground-form. Its relation to the primitively triradiate tripod presents a still unsolved problem, and the numerous relations of the zygopleural to the multiradiate ground-forms in the NASSELLARIA are exceedingly complicated. The zygopleural ground-form is less widely distributed among the PHÆODARIA, though it is very characteristically developed in the rich and varied group of Challengerida (Pl. 99). (See Gener. Morphol., Bd. i. pp. 507-527.) 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_, _Tetraplagia_, _Tetraplecta_, &c. II. CENTRAXONIA. {xix} The geometrical centre of the body is a straight line (the vertical main axis).