REPORT ON THE RADIOLARIA. 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 D i s c o i d e a 

 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 PH^ODARIA 

 (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 

 (Pis. 13-17, and 39), as well as most Belonaspida. (See Gener. Morphol., Bd. i. 

 p. 422.) 



32. Allopolar-Monaxon or Conotypic Ground-Form. The allopolar uniaxial or 

 conotypic ground-form is characterised by the possession of a vertical main axis whose 

 two poles are unlike, while no transverse axes are differentiated. All horizontal planes 

 cutting the main axis at right angles are circles, and decrease more rapidly from the 

 largest plane towards the basal than towards the apical pole. The most important 

 ground-forms of this group are the ovoid, the cone, and the hemisphere. They often 

 occur (and in geometrical perfection) in the egg-shaped central capsule and podoconus of 

 the NASSELLARIA, as well as in the shells of several groups of this legion, particularly in 

 the Cyrtocalpida or Monocyrtida eradiata (PL 51, figs. 10-13), and in many Stichocyrtida 

 eradiata ; furthermore, they are also seen among the PH^ODARIA, e.g., certain Chal- 

 lengerida (PI. 99, figs. 19-22). (See Gener. Morphol., Bd. i. p. 426.) 



33. The Regular Dipyramidal or Quadrilonchial Ground-Form. The ground-forms 

 whose geometrical type is the regular double pyramid are characterised by a vertical 

 main axis which possesses equal poles, and which is crossed at its centre by several 

 equal transverse axes. The horizontal equatorial plane is therefore a regular polygon, 

 and divides the body into two equal regular pyramids. The simplest and commonest 

 form of this group is the quadratic octahedron, the ground-form of the quadratic crystal- 

 lographic system '; its equatorial plane is a square. This regular dipyramidal ground- 

 form occurs among the SPUMELLARIA in the shells of the Staurosphserida as well as of 

 many D i s c o i d e a, in which several equidistant radial spines or arms lie in the quadratic 

 equatorial plane of the body, and project from the margin of the lenticular disc (e.g., 

 Sethostaurus, PI. 31 ; Histiastrum, PI. 46, &c.). It is, however, among the ACANTHARIA 

 that the most important part is played by this ground-form (and especially by the quadratic 

 octahedron) ; it forms the basis of all those Acanthometra and Acanthophracta 

 in which twenty radial spines are disposed according to the Miillerian Law, and in which 



