Richard Hertwig in his excellent work (Der Organismus der Radiolarien, 1879, p. 25) separated his "Acanthophractida" perfectly from the "Ommatida" (or the siliceous Sphæroidea), and united them with the "Acanthometrida" in his order "Acanthometrea." But he separated them also from the nearly allied Diploconida, following my former arrangement. He distinctly noted that the skeleton in all these Acanthophractida (as well as in the Acanthometrida) consists not of silex but of the organic substance "acanthin."

The astonishing number of new and interesting forms of Acanthophracta which I have found in the rich collection of the Challenger enables me to distinguish now in this suborder six different families, two of which are perfectly new (the Sphærocapsida and the Hexalaspida). But the four other families also are so much enlarged that their interesting morphology appears in quite a new and clear light. Far the largest and most important of these six families is that of the true Dorataspida, which embraces seventeen genera and one hundred and eight species (more than the other five families together). From this largest and oldest ancestral family four other families have afterwards arisen, whilst a single family, the Sphærocapsida, seems to possess no direct phylogenetic connection with the five other families.

The peculiar and quite new family of Sphærocapsida (Pl. [133], figs. 7-11; Pl. [135], figs. 6-10) differs from all other Acanthophracta in the singular structure of the spherical acanthinic shell, composed of innumerable small plates or aglets, each of which is pierced by a very small porule. This peculiar pavemented shell (enclosing the central capsule and separated from it by the jelly-like calymma) seems to be produced on the surface of the spherical calymma, immediately by secretion of the pseudopodia, and independently from the twenty radial spines, united in the centre of the sphere. On the twenty points, where the spines perforate the shell, there are originally eighty larger pores (four around each piercing spine); but there is no certain indication that the shell is produced by the meeting apophyses of the twenty spines, as is the case in the five other families of Acanthophracta. Therefore perhaps it is more natural to unite these latter into another suborder as Cladophracta, and to separate them from the Sphærocapsida, which may be called Capsophractæ.

The Dorataspida (Pls. [134]-[138]), the common ancestral stock of the Cladophracta, in the definition here restricted embraces all those Acanthophracta in which the spherical lattice-shell is simple and composed of the meeting branches of twenty radial spines united in its centre. As already pointed out above, this family is probably diphyletic, and embraces two subfamilies which have been derived originally from two different forms of Acanthonida—the Diporaspida (with two opposite apophyses on each spine) derived from the Phractacanthida, and the Tessaraspida (with four crossed apophyses on each spine) derived from the Stauracanthida; in the former we find originally forty apophyses, in the latter eighty apophyses, by the meeting branches of which the spherical lattice-shell originates. The four following families of Acanthophracta have probably been derived from the Diporaspida.

The Phractopeltida (Pl. [133], figs. 1-6) differ from all other Acanthophracta in the possession of a double lattice-shell, composed of two concentric spheres which are united by the twenty radial spines meeting in the centre. As all Phractopeltida possess originally only two apophyses on each radial spine, they must be derived from the Diporaspida (Orophaspis), and bear to them the same relation as the Dyosphærida do to the Monosphærida. As the spherical central capsule of the Phractopeltida is enclosed between both shells, smaller than the outer, larger than the inner shell, the latter may be called "medullary shell," the former "cortical shell." This family represents among the Acanthophracta only the "Diplophracta," whilst all others are "Haplophracta."

The three families here characterised may be called together "Sphærophracta," as their central capsule and the enveloping shell are constantly spherical (or the shell sometimes an "endospherical polyhedron"). On the contrary the following three families of Acanthophracta may be united as "Prunophracta," as their central capsule and shell are never spherical, but either ellipsoidal or lenticular or of another form. The common ancestral stock of this suborder are the Belonaspida, in which the form of the central capsule and the enclosing lattice-shell is ellipsoidal; they are derived from the Dorataspida (and probably all from the subfamily Diporaspida) by the prolongation of two opposite radial spines which are larger than the eighteen others; they are the two equatorial spines of the "hydrotomical axis" (compare above, p. [719], and Pl. [136], figs. 6-9).

The Hexalaspida (Pl. [139]) represent a new and very remarkable family, distinguished from all other Acanthophracta by the preponderating development of six stout radial spines, which are much larger than the fourteen others. These six principal spines lie in one meridian plane of the shell (in the "hydrotomical plane," p. [720]), and are the two opposite equatorial spines and the four appertaining polar spines of the same plane. As the fourteen smaller spines develop their apophyses at smaller distances from the centre, the shell assumes a peculiar lenticular or discoidal form, and the margin of this disk bears the six larger spines. Moreover the enclosed small central capsule is lenticular. The Hexalaspida may be derived immediately from the Belonaspida.

The Diploconida (Pl. [140]) form the last and the most modified family of all Acanthophracta. The remarkable shell exhibits the strange form of a double cone, bearing in its axis two very large opposite spines; these are the two equatorial spines of the "hydrotomical axis" (p. [719]). The double-conical or nearly cylindrical shell is composed of three different parts or segments; the small middle part is the true lattice-shell of the Hexalaspida and Belonaspida, and bears the eighteen smaller (often quite rudimentary) radial spines. The two other parts (opposite on both poles of its hydrotomical axis) are the conical or cylindrical, solid, basal sheaths of the two large equatorial spines, enveloping their major part. In consequence of this peculiar metamorphosis of the shell the Diploconida represent the last and the most aberrant group of all Acantharia.

Synopsis of the Suborders and Families of Acanthophracta.

Suborder I. SPHÆROPHRACTA.

Twenty radial spines of equal size. Shell spherical (or an endospherical polyhedron).

Shell spherical, simple, pierced by twenty or eighty aspinal pores and composed of a pavement of innumerable very small plates or aglets, each pierced by one porule,1. Sphærocapsida.
Shell spherical, simple, composed of the meeting branches of two or four apophyses of the twenty radial spines,2. Dorataspida.
Shell spherical, double, composed of two concentric lattice-spheres, which are connected by the twenty radial spines and composed of the meeting branches of their apophyses,3. Phractopeltida.

Suborder II. PRUNOPHRACTA.

Twenty radial spines of unequal size; two or six hydrotomical spines much larger than the eighteen or fourteen others. Shell not spherical.

Shell ellipsoidal, with prolonged hydrotomical axis, the two spines of which are larger than the eighteen others,4. Belonaspida.
Shell lenticular or discoidal, with six larger spines placed in the hydrotomical plane (fourteen other spines much smaller),5. Hexalaspida.
Shell diploconical or nearly cylindrical, with two opposite large funnels, the sheaths of the enlarged two spines of the hydrotomical axis (eighteen other spines much smaller or rudimentary),6. Diploconida.