_Dimensions._--Breadth of the mitral ring 0.04, of the basal ring 0.08. _Habitat._--Central Pacific, Station 271, depth 2425 fathoms. {1011}3. _Pseudocubus hexapylus_, n. sp. (Pl. 94, fig. 12). Mitral ring circular, two-thirds as broad as the subcircular, or nearly square, basal ring; both rings with a simple spine on the two sagittal corners (dorsal and ventral), with a forked or branched larger spine on the two lateral corners (right and left). All twelve rods more or less curved and thorny. _Dimensions._--Breadth of the mitral ring 0.07, of the basal ring 0.1. _Habitat._--Central Pacific, Station 272, depth 2600 fathoms. Genus 438. _Lithocubus_,[56] Haeckel, 1881, Prodromus, p. 447. _Definition._--#Tympanida# with two simple horizontal rings of equal size, connected by four parallel vertical columellæ. The genus _Lithocubus_ is very remarkable for the regular cubical form of the skeleton, which is composed of twelve siliceous rods, corresponding to the twelve edges of a cube. Eight of these are horizontal, and enclose the two parallel square horizontal gates, the superior mitral and the inferior basal gate. The four other rods are vertical, parallel, and connect (as lateral edges of the cube) the corresponding corners of the two horizontal squares. Two opposite ones of these four vertical columellæ are the remaining halves of the incomplete sagittal ring; the two alternate are the remaining halves of the incomplete frontal ring. The four lateral gates between these four columellæ are also square or rectangular, and either of the same size as the two horizontal gates, or somewhat larger. _Lithocubus_ may be derived from _Acrocubus_ by reduction of the coryphal and the basal part of the primary sagittal ring, the only remaining parts of which are the anterior and the posterior columellæ. 1. _Lithocubus geometricus_, n. sp. (Pl. 94, fig. 13). The twelve rods, corresponding to the edges of the geometrical cube, are straight and smooth; the eight corners provided with a small conical thorn. The shell exhibits six equal square sides, and represents therefore exactly the geometrical form of a cube. _Dimensions._--Diameter of the cube 0.05; thickness of the bars 0.008. _Habitat._--Tropical Pacific, Station 224, depth 1850 fathoms. 2. _Lithocubus octacanthus_, n. sp. The twelve rods of the cubical shell are slightly curved, convex, smooth, as in the similar preceding species. It differs from that in the development of eight slender radial spines, arising from the eight corners of the geometrical cube, from two to three times as long as its diameter, and lying opposite in pairs in its diagonals. _Dimensions._--Diameter of the cube 0.06; length of the spines 0.15. _Habitat._--North Atlantic, Canary Islands, surface. {1012}3. _Lithocubus vinculatus_, Haeckel. _Acanthodesmia vinculata_, J. Müller, 1856 (_partim_), Abhandl. d. k. Akad. d. Wiss. Berlin, Taf. i. fig. 7 (not 4-6).