Galear and thoracal bows forked, the former smaller than the frontal bows, the latter larger. Sagittal constriction rather flat on the apical pole, which bears a vertical forked horn, very deep on the basal pole of the primary ring. All bows armed with numerous spines, which are partly simple, partly forked, and mainly developed at the basal part of the thoracal bows.
Dimensions.—Length of the shell 0.2, breadth 0.2.
Habitat.—Central Pacific, Station 271, depth 2425 fathoms.
Genus 428. Microcubus,[[47]] Haeckel, 1881, Prodromus, p. 447.
Definition.—Tympanida with two bisected horizontal rings, connected by four vertical columellæ, which are bisected again by an equatorial ring.
The genus Microcubus is derived from its ancestral form Acrocubus by development of a third horizontal ring, which lies in the equatorial plane, parallel to the upper mitral and the lower basal ring. Whilst these latter are both bisected by the complete vertical sagittal ring, the equatorial ring is complete, and connected at the poles of the sagittal axis with the principal ring, and at the poles of the transverse axis with the frontal ring. Therefore the shell exhibits not eight large gates as in Acrocubus, but twelve, viz., four horizontal gates (two superior mitral and two inferior basal), four upper vertical gates (between the mitral and the equatorial ring), and four lower vertical gates (between the basal and the equatorial ring).
1. Microcubus dodecastoma, n. sp. (Pl. [94], fig. 9).
All twelve gates simple, irregularly ovate or pentagonal. Frontal ring on the sides concave, constricted by the equatorial ring. All rings curved, armed with short irregular thorns. No larger basal spines.
Dimensions.—Length of the shell 0.13, breadth 0.15.
Habitat.—Central Pacific, Stations 271 to 274, depth 2350 to 2750 fathoms.