| 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. | ||||
Suborder I. SPHÆROPHRACTA, Haeckel.
Definition.—Shell spherical, with twenty radial beams of equal size.
Family XXXIX. Sphærocapsida, Haeckel (Pl. [133], figs. 7-11; Pl. [135], figs. 6-10).
Sphærocapsida, Haeckel, 1881, Prodromus, p. 469.
Definition.—Acantharia with simple spherical porous shell, composed of innumerable very small plates, each of which is pierced by one radial porule. Twenty radial spines of equal size meeting in the centre of the shell and disposed according to the Müllerian law of the Icosacantha, sometimes short and enclosed in the shell, at other times long and piercing it (rarely rudimentary or quite absent). Shell pierced therefore either by twenty larger perspinal pores or by eighty smaller aspinal pores. Central capsule spherical, enclosed in the porous shell.
The family Sphærocapsida, founded by me in 1881 for the single genus Sphærocapsa, represents a very peculiar and remarkable group of the Acanthophracta, very different from the five other families of this suborder, and probably derived, independently of them, directly from the Acanthonida. Whilst the lattice-shell of the five other families is composed of the meeting branches of lateral apophyses of the twenty spines, and its meshes are all or partly the intervals between these apophyses, in the Sphærocapsida the spherical shell has quite another structure, and is composed of innumerable small plates (each with one pore) which are secreted on the surface of the spherical calymma, independently of the twenty radial spines, which do not possess true apophyses.
In all Sphærocapsida the structure of the spherical shell is quite peculiar and different from that of all other Radiolaria. It is composed everywhere of innumerable very small plates or aglets, which are connected irregularly like paving-stones, and form a single continuous layer or pavement on the surface of the spherical calymma (Pl. [133], fig. 11, a; Pl. [135], figs. 8, 10). The small plates or paving-stones, which we will call "aglets," are connected at their meeting edges by a kind of cement, and form together with it a continuous thick capsule of acanthin. The form of the aglets is commonly more or less irregular, roundish or polygonal, sometimes longish (Pl. [133], fig. 11, a), more rarely it becomes rather regular, hexagonal, square, or roundish (Pl. [135], fig. 8). Usually all aglets of one and the same individual are of nearly equal size, between 0.01 and 0.02 in diameter, rarely less or more. The outer face of the aglets is more or less concave, so that the elevated meeting edges of the neighbouring aglets commonly form together a prominent network of crests (Pl. [135], figs. 8, 10); rarely the meeting edges partly cover one another like squamules (Pl. [133], fig. 11, a). Each small plate or aglet is pierced in its centre by a single radial canalicule or porule. The dimpled surface, so produced, resembles somewhat the dimpled plates of Ceriaspis, &c. Different from these innumerable very small dimples of the surface are the twenty larger "spinal dimples," or the concave larger plates, which are originally pierced by the twenty radial spines. Before we describe these, we must examine the spines themselves.
The twenty radial spines of all observed Sphærocapsida (sixteen species) agree perfectly with those of the genus Acanthonia (p. [749]), and especially with Acanthonia tetracopa, Acanthonia denticulata, &c. All twenty spines, regularly disposed according to the Müllerian law of the Icosacantha, are of equal size, constantly four-edged prismatic, of equal breadth throughout their whole length. The prominent four edges are parallel, sometimes smooth, at other times elegantly denticulated. The central bases of the twenty spines are pyramidal, without leaf-cross, and propped one upon another with their triangular faces, as in the majority of the Acanthonida.