REPORT ON THE RADIOLARIA. CXV 



sjiine, and the Stauraeautliida, with four crossed apophyses on each radial s^^ine. The 

 tliree genera of the Zygacauthida represent tlie stem-forms of the three families, since the 

 radial spines in AcantJwmetron (the most primitive form of Acanthonida) are 

 cylindrical, in Zygacantha two-edged, and in Acanthonia four-edged (p. 741). 



175. Capsophracta and Cladophracta. — The extensive order A c a n t h o p h r a c t a, 

 which embraces all Acantharia with a complete lattice-shell, is polyphyletic, its main 

 subdivisions have been developed independently from different branches of the 

 Acanthonida. The whole order may be divided directly into two main groups, 

 the Capsophracta and C 1 a d o p h r a c t a (p. 793), which differ in the structure and 

 the origin of their lattice-shell. The group (or suliorder) Capsophracta includes only 

 the single family Sphasrocapsida (p. 795, PL 133, figs. 7-11; PI. 135, figs. 6-10); 

 the lattice-shell arises independently of the twenty radial spines, being made up like a 

 pavement of innumerable small acanthin plates, united by a kind of cement ; each plate 

 being perforated by a fine pore. In addition twenty larger main pores (or groups of four 

 pores each) are present, corresponding to the twenty radial spines; these are always equal, 

 quadrangular prismatic, without transverse processes as in Acanthonia. In the 

 Cladophracta, which include the five remaining families of the Acanthophracta, 

 the structure and origin of the lattice-shell are quite different ; the lattice-shell is here 

 made up of the branches of the transverse processes, which radiate tangentially from the 

 twenty radial spines and are only united secondarily. 



1 7(3. Ascent of the Dorataspida. — The group Cladophracta, or those Acanthauia 

 whose lattice-shell arises by the union of transverse processes of the twenty radial spines, 

 includes five different families, whose stem-group is the family Dorataspida, with a 

 simple spherical lattice-shell. This family itself is, however, diphjdetic in origin, being 

 composed of two essentially and originally different subfamilies — Diporaspida and 

 Tessaraspida (p. 803). The Diporaspida (p. 808, Pis. 137, 138) have been developed 

 from the Phractacanthida, and as each radial spine of the latter bears two opposite 

 apophyses, so the lattice-shell of the former has forty primary aspinal pores (two on the 

 base of each spine). On the other hand, the Tessaraspida (p. 830, Pis. 135, 136) have 

 been developed from the Stauracanthida, and as each radial spine of the latter bears four 

 crossed apophyses, so the lattice-shell of the former has eighty primary aspinal pores (four 

 at the base of each spine). 



177. Descent of the Diporaspida. — Whilst the Tessaraspida (§ 176) have given rise 

 to no new groups which could take rank as independent families, no less than four 

 separate families of Acanthaeia have arisen from the Diporaspida. The Phractopeltida 

 (PL 133, figs. 1-6) are distinguished from all other Acanthakia by the possession of 

 two concentric spherical lattice-shells, and have probably been developed from the 



