The genus Diplocolpus is the last and the most modified genus among the Acantharia. The eighteen smaller spines of Diploconus are here quite rudimentary or have even perfectly disappeared by atrophy, so that the shell seems to consist only of the large hydrotomical spines and their enveloping sheaths.
Subgenus 1. Diplocolpulus, Haeckel.
Definition.—Mantle of the double cone not compressed; its transverse section therefore circular.
1. Diplocolpus costatus, n. sp. (Pl. [140], fig. 7).
Mantle of the double cone thin walled, not compressed; its transverse section circular or nearly hexagonal; its contour little convex; surface with six thick longitudinal equidistant ribs and many thinner ribs between them; margin denticulate. Transverse diameter of the shell one-third as long as the longitudinal. The two hydrotomical spines thick and short, little prominent over the polar mouth. No rudiments of other spines visible externally.
Dimensions.—Length of the shell 0.15; equatorial and polar breadth 0.05.
Habitat.—Indian Ocean (Madagascar), Rabbe, surface.
2. Diplocolpus cristatus, n. sp. (Pl. [140], fig. 6).
Mantle of the double cone thick walled, not compressed; its transverse section circular or hexagonal; its contour nearly straight; surface with six thick prominent longitudinal equidistant ribs; margin irregularly dentated. Transverse diameter of the shell half as long as the longitudinal. The two hydrotomical spines very thick and short, pyramidal, little prominent. Short rudiments of the eighteen other spines present, bristle-shaped.
Dimensions.—Length of the shell 0.1, breadth 0.05.