Mitral ring triangular, somewhat smaller than the triangular basal ring, both connected at the corners by three vertical, slightly curved columellæ. From the six corners arise six radial spines of different size and form; the two paired spines of the mitral ring are simple and very small, whilst those of the basal ring are three times as long and bear a lateral branch. The odd spine of the third corner is on both rings larger, and distinguished by some thorns. This difference seems to indicate that these two odd spines correspond to the apical horn and the caudal foot of _Cortina_, and the columella between them is the rest of the reduced sagittal ring. In this case the quadrangular vertical ring, which is composed of the two paired columellæ and the two connecting horizontal rods, would be probably the frontal ring. _Dimensions._--Length of the shell 0.09, breadth 0.07. _Habitat._--Equatorial Atlantic, Station 347, surface. {1010}2. _Prismatium tripleurum_, Haeckel. _Prismatium tripleurum,_ Haeckel, 1862, Monogr. d. Radiol., p. 270, Taf. iv. fig. 6. _Acanthodesmia prismatium_, Haeckel, 1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 799. Mitral and basal rings of equal size, equilateral triangular, both connected at the corners by three vertical equal columellæ. From the six corners arise six short, equal, conical radial spines. The nine rods of the shell correspond to the nine edges of a triangular equilateral prism. _Dimensions._--Length of the shell 0.044, breadth 0.032. _Habitat._--Mediterranean (Messina), surface. Genus 437. _Pseudocubus_,[55] n. gen. _Definition._--#Tympanida# with two simple horizontal rings of different sizes, connected by four divergent columellæ. The genus _Pseudocubus_ exhibits the characteristic form of an obelisk, or a truncated four-sided pyramid. The four edges of this obelisk, or the four "columellæ," are the remaining parts of the two incomplete vertical rings; two opposite edges are the dorsal and ventral rod of the sagittal ring; the two alternate edges between these are the lateral rods of the frontal ring. The two horizontal rings, which become connected by the four divergent columellæ, are the smaller mitral ring and the larger basal ring; both are either square or nearly circular. 1. _Pseudocubus obeliscus_, n. sp. (Pl. 94, fig. 11). Mitral ring square, with circular gate and two divergent simple spines on each corner. Basal ring twice as broad, square, with four convex sides and one simple spine on each corner. Four columellæ straight. All twelve rods with three smooth edges. _Dimensions._--Breadth of the mitral ring 0.02, of the basal ring 0.05. _Habitat._--Central Pacific, Station 266, depth 2750 fathoms. 2. _Pseudocubus octostylus_, n. sp. Mitral ring square, with one stout pyramidal spine on each corner. Basal ring twice as broad, square, also with a single strong spine on each corner. Four columellæ straight. All twelve rods with three smooth edges.