Mantle of the double cone thin walled, not compressed; its transverse section circular; its contour strongly convex; its surface with six stronger and many smaller ribs; its margin with numerous straight and long, parallel denticles. Diameter of its mouth half as long as the whole shell and one and a half times as long as its equatorial diameter. The two principal spines one-fourth longer than their conical sheath. The eighteen smaller spines about half as long, thin, straight. Both geotomical spines thick and short.
Dimensions.—Length of the shell 0.2; equatorial breadth 0.07, polar breadth 0.1.
Habitat.—West Tropical Pacific, Station 225, surface.
3. Diploconus cylindrus, n. sp.
Mantle of the double cone thick walled, dark, cylindrical, not compressed; its transverse section circular; its contours straight, parallel; its surface with strong parallel straight longitudinal ribs; its margin irregularly dentated. Diameter of its mouth one-fifth as long as the whole shell and two-thirds as long as the diameter of the equatorial intumescence. The two principal spines prismatic, nearly twice as long as their cylindrical sheath. The smaller spines about half as long, thin, conical, straight. (Resembles the medial part of Hexacolpus trypanon, Pl. [140], fig. 11.)
Dimensions.—Length of the shell 0.25; equatorial breadth 0.08, polar breadth 0.05.
Habitat.—North Pacific, Station 241, surface.
Subgenus 2. Diploconium, Haeckel.
Definition.—Mantle of the double cone compressed from both poles of the shortened geotomical axis; its transverse section therefore elliptical.
4. Diploconus fasces, Haeckel.