Genus 380. Diploconus,[[420]] Haeckel, 1862, Monogr. d. Radiol., p. 404.
Definition.—Diploconida with two very large spines (opposite in the hydrotomical axis) and ten to eighteen other much smaller spines externally visible.
The genus Diploconus must be derived from Hexacolpus (among the Hexalaspida) by the stronger development of the two hydrotomical spines and their large sheaths; all the other eighteen spines are much smaller, and usually devoid of prominent sheaths. Sometimes the eight tropical spines are rudimentary.
Subgenus 1. Diploconulus, Haeckel.
Definition.—Mantle of the double cone not compressed; its transverse section therefore circular.
1. Diploconus amalla, n. sp. (Pl. [140], fig. 1).
Mantle of the double cone thick walled, not compressed, its transverse section circular; its contour little convex; its surface nearly smooth; its distal margin regularly denticulated. Diameter of its mouth twice as large as the equatorial diameter of the shell and one-third as long as its total length. The two large spines prismatic, one-fourth longer than their conical sheath. The eighteen smaller spines very thin, about half as long as the former, a little curved.
Dimensions.—Length of the shell 0.3; equatorial breadth 0.05, polar breadth 0.1.
Habitat.—Central Pacific, station 271, surface.
2. Diploconus cyathiscus, n. sp. (Pl. [140], fig. 3).