Habitat.—Central Pacific, Station 268, depth 2900 fathoms.
Genus 426. Acrocubus,[[45]] Haeckel, 1881, Prodromus, p. 447.
Definition.—Tympanida with two bisected horizontal rings, connected by four vertical columellæ. No equatorial ring.
The genus Acrocubus is the most primitive of those remarkable Tympanida, the skeleton of which represents more or less exactly the form of a geometrical cube. A simple complete sagittal ring bears on the superior and on the inferior part two opposite pairs of lateral branches; by union of the convergent branches of each side there arise two horizontal parallel quadrangular rings, which are bisected by the sagittal ring. Therefore the superior ring encloses two triangular mitral gates, the inferior two triangular basal gates. The lateral corners of the two gates of each side are connected by a vertical rod or columella. These two parallel columellæ are the remaining middle parts of the secondary or frontal ring, which is incomplete on the superior and on the inferior face of the body. If the sagittal ring also become incomplete, by the loss of the superior and inferior part (the anterior and posterior only remaining), then Acrocubus is transformed into Lithocubus.
Subgenus 1. Apocubus, Haeckel.
Definition.—Basal ring without descending feet.
1. Acrocubus octopylus, n. sp. (Pl. [82], fig. 9).
Mitral ring somewhat smaller than the basal ring; both rings rhombic, with curved outlines. Sagittal ring elliptical, with six pairs of nodulate protuberances. Four columellæ curved. Nodal points without radial spines.
Dimensions.—Height of the frontal ring 0.12, breadth 0.18.
Habitat.—Western Tropical Pacific, Station 225, depth 4475 fathoms.