Page:Scientific results HMS Challenger vol 18 part 1.djvu/135

Rh spine, and the Stauracanthida, with four crossed apophyses on each radial spine. The three genera of the Zygacanthida represent the stem-forms of the three families, since the radial spines in Acanthometron (the most primitive form of ) are cylindrical, in Zygacantha two-edged, and in Acanthonia four-edged (p. 741).

175. Capsophracta and Cladophracta.—The extensive order, which embraces all with a complete lattice-shell, is polyphyletic, its main subdivisions have been developed independently from different branches of the. The whole order may be divided directly into two main groups, the and  (p. 793), which differ in the structure and the origin of their lattice-shell. The group (or suborder) includes only the single family Sphærocapsida (p. 795, Pl. 133, figs. 7-11; Pl. 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, which include the five remaining families of the , 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.

176. Ascent of the Dorataspida.—The group, or those 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, diphyletic in origin, being composed of two essentially and originally different subfamilies—Diporaspida and Tessaraspida (p. 803). The Diporaspida (p. 808, Pls. 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, Pls. 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 have arisen from the Diporaspida. The Phractopeltida (Pl. 133, figs. 1-6) are distinguished from all other by the possession of two concentric spherical lattice-shells, and have probably been developed from the