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. |
