998 THE VOYAGE OF H.M.S. CHALLENGER. 



The genus Microcuhus is derived from its ancestral form Acrocubus by develop- 

 ment 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 jirincipal 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. (PI. 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 O'l, breadth 0'12. 



Habitat. — Fossil in Barbados. 



8. Microcubus zonarius, n. sp. (PI. 94, fig. 8). 



All twelve gates simple, ovate. Frontal ring on the sides conve.x, not constricted Ijy 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 quadrujics, 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 



