XIV THE VOYAGE OF H.M.S. CHALLENGER. 



these Spliseroidea tlie tyj^ical ground-form is usually indicated by six equal radial spines, 

 which are opposed to each other in pairs, and lie in three similar axes perpendicular to 

 each other ; these are the three axes of the tesseral crystallographic system ; one of 

 them is vertical, whilst the other two cross each other at right angles in its centre. 

 Occasionally, too, the spherical form of the lattice-shell passes over into that of the 

 regular octahedron (PI. 22, figs. 8, 10). The same form recurs in Circoporus (PL 117, 

 fig. 6) among the Ph^odaria. In the vegetable kingdom it is exhibited by the 

 antheridia of Chara. It is not found in the Nassellakia and Acanthaeia. (See Gener. 

 Morphol., Bd. i. p. 412.) 



29. Tlie Regular Cubic Ground- FovTn. — The ground -form whose geometrical type is 

 that of a die or cube, is actually presented in a very striking manner by various 

 Radiolaria. Among the Spumellaria it occurs in certain Sphser oidea, e.^., in the 

 Astrosphserid genera Centrocuhus and Octodendrum (PI. 18, figs. 1-3); in these the 

 central medullary shell is a complete cube, bounded by six equal squares, from the 

 eight angles of which eight equal radial spines project. This form can also be regarded 

 as present in those Sphseroidea whose spherical lattice-shell bears eight equal and 

 equidistant radial spines (many Astrosphserida). Besides these the cubic ground-form is 

 to be seen in certain Nassellaria of the family Tympanida, especially in Lithocuhus 

 (PI. 82, fig. 12 ; PI. 94, fig. 13), in many species of Acrocubus, 3Iicrocubus, &c. ; the 

 twelve bars of its lattice-skeleton correspond often exactly to the edges of the cube. 

 (See Gener. Morphol, Bd. i. p. 413.) 



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 B e 1 o i d e a, and especially in those members of the 

 Thalassosphserida and Sphaerozoida 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 P 1 e c t o i d e a, 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 B e 1 i d e a, 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. Tlie Isopolar-Alonaxon or Phacotypic Grvund-Form. — The isopolar uniaxial or 

 phacotypic ground-form is characterised by the possession of a vertical main axis with 



