{998}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. 2. _Microcubus pentacircus_, n. sp. All twelve gates simple. Frontal ring on the sides convex, not constricted by the equatorial ring. All rings tuberculate, with irregular roundish knots. No larger basal spines. Similar to the preceding species; but the five rings are much thicker and tuberculate (not thorny); the twelve gates are therefore relatively smaller. _Dimensions._--Length of the shell 0.1, breadth 0.12. _Habitat._--Fossil in Barbados. 3. _Microcubus zonarius_, n. sp. (Pl. 94, fig. 8). All twelve gates simple, ovate. Frontal ring on the sides convex, not constricted by the equatorial ring, nearly twice as broad as long. All rings thorny, very thin, except only the broad angular sagittal ring. No larger basal spines. _Dimensions._--Length of the shell 0.08, breadth 0.15. _Habitat._--Central Pacific, Station 271, depth 2425 fathoms. 4. _Microcubus quadrupes_, n. sp. All twelve gates simple, square. Frontal ring square, not constricted by the equatorial ring. All rings smooth, quadrangular. Basal ring with four short vertical conical feet, two of which (right and left) are inferior prolongations of the frontal ring, and arise from the lateral corners of the {999}basal gates; two others are prolongations of the sagittal ring (sternal and caudal), and arise from the medial corners of the basal gates. (The shell is like a small cube with four feet.) _Dimensions._--Length of the shell 0.13, breadth 0.11. _Habitat._--North Pacific, Station 253, depth 3125 fathoms. 5. _Microcubus cornutus_, n. sp.