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



(PL 8, fig. 2), and the absolutely irregular shells of the Phorticida and Soreumida among 

 the Larco idea. (See Gener. Morphol., Bd. i. p. 400.) 



23. The Subsidiary Grouj^s of Geometrical Ground- Forms. — The four natural principal 

 groups of ground-forms, which have just been defined according to the nature of the centre 

 of their bodies, may be divided again into numerous subsidiary groups, defined by the 

 relations of the constant axes and the two poles of each axis, as well as by the number of 

 the axes and the difi"erentiation of the secondary with respect to the principal axis. The 

 most important of these subsidiary groups into which the principal ones are immediately 

 divided are the following: — (l) The Centrostigma (or sphserotypic) are divided into 

 spheres (Homaxonia) and endospherical polybedra (Polj^axonia). (2) The Centraxonia 

 (or grammotypic) into uniaxial (Monaxonia) and those with crossed axes (Stauraxonia) ; 

 among the former of these may be distinguished the isopolar (phacotypic) and the allopolar 

 (conotypic); among the latter the double and single pyramids. (3) The Centroplana (or 

 bilaterals) are divided into amphipleura (or bilaterally radial) and zygopleura (or bilaterally 

 symmetrical). (4) The Acentrica (or Anaxonia) or absolutely irregular ground-forms, 

 present no special subdivisions. 



For a complete system of the geometrical ground-forms and their relation to promorishological 

 classification, see Gener. Morphol., Bd. i. pp. 555-558. 



24. The Spherical or Homaxon Ground-Form. — The spherical is the only absolutely 

 regular ground-form, since only in it are all axes which pass through the centre equal ; 

 it is very often realised among the Radiolaria, especially in the Spumellaria and 

 AcANTHARiA, where it furnishes the common original ground-form, but it is often to be 

 seen in the shells of many Ph.i:odaria (in most P h se o s p h 8b r i a); on the other hand, 

 it is never found among the Nassellaria. Geometrical spheres, in the strict sense of 

 the term, are only to be found among the Spumellaria and Acanthaeia, namely, in the 

 central capsule of many C o 1 1 o d a r i a (Pis. 1,2) and all S p h as r o i d e a (Pis. 1 1-30) 

 as well as of many Acanthometra and Acanthophracta (Pis. 128-138). Nevertheless, 

 speaking generally, one includes those central capsules and skeletons which have been 

 distinguished here as endospherical polyhedra. (On these ground-forms see Gener. 

 Morphol, Bd. i. pp. 404-406.) 



25. Tlie Endospherical Polyhedral Gh^ound-Form. — The endospherical polyhedron 

 or polyaxon ground-form naturally follows the spherical or homaxon. Under it are 

 included all polyhecba whose angles fall in the surface of a sphere ; this ground-form is 

 especially common among the Spumellaria, especially in the shells of S p h se r o i d e a, 

 but is also found among the Acantharia (especially in the Astrolophida and S p h ae r o- 

 p h r a c t a), as well as among the P h se o s p h se r i a (in most genera of the Orosphserida, 

 Sagosphserida, and Aulosphserida). Strictly speaking, all those lattice-shells which have 



