Page:Scientific results HMS Challenger vol 18 part 1.djvu/1095

Rh denticulate incisions. Diameter of its mouth one-third as long as the whole shell and somewhat smaller than the equatorial diameter. Hydrotomical spines about twice as long as their sheath, pyramidal, with emarginate point and six wings. Geotomical spines short and strong, pyramidal. Sixteen other spines thin, straight, conical.

Dimensions.—Length of the shell 0.3; equatorial breadth 0.11, polar breadth 0.1.

Habitat.—Central Pacific, Station 266, surface.

Definition.— with only two developed, very large spines (opposite in the hydrotomical axis); all the other eighteen spines quite rudimentary or externally atrophied.

The genus Diplocolpus is the last and the most modified genus among the. 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.

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.