_Dimensions._--Diameter of the rings 0.09, length of the columellæ 0.05. _Habitat._--North Pacific, Station 236, surface. 3. _Parastephanus asymmetricus_, n. sp. (Pl. 82, fig. 10). Two horizontal rings thorny, of equal size, irregularly ovate or nearly semicircular, and of somewhat asymmetrical form. The two connecting parallel columellæ are straight, cylindrical, about as long as the diameter of the rings. The peculiar asymmetry in the form of the rings raises the possibility that the two columellæ in this species are the remaining parts of a frontal ring, and that the sagittal ring has altogether disappeared. In this case the convex half of both rings would be the ventral, the straight half the dorsal part. _Dimensions._--Diameter of the rings 0.1, length of the columellæ 0.08. _Habitat._--Central Pacific, Station 272, depth 2600 fathoms. {1009}4. _Parastephanus dispar_, n. sp. Two horizontal rings elliptical, thorny, of different sizes, the basal ring one and a half times as broad as the mitral ring. The two connecting columellæ are curved, somewhat longer than the diameter of the mitral ring. _Dimensions._--Diameter of the basal ring 0.09, mitral 0.06; length of the columellæ 0.07. _Habitat._--South Pacific, Station 300, depth 1375 fathoms. Genus 436. _Prismatium_,[54] Haeckel, 1862, Monogr. d. Radiol., p. 270. _Definition._--#Tympanida# with two simple horizontal rings, connected by three vertical columellæ. The genus _Prismatium_ was hitherto the only known form of the Tympanida, and the only species described in 1862 in my Monograph was _Prismatium tripleurum_ (_loc. cit._, p. 270, pl. iv. fig. 6). A new species from the Challenger collection (Pl. 93, fig. 22), _Prismatium tripodium_, explains the true structure of this remarkable genus. The skeleton is composed of nine siliceous rods, corresponding to the edges of a trilateral prism. Six of these are horizontal, and enclose the two parallel triangular horizontal gates, the superior of which is formed by the mitral or coryphal, and the inferior by the basal or cortinar ring. The three other rods are vertical and nearly parallel, and connect (as lateral edges of the prism) the corresponding corners of the two parallel horizontal triangles. Two of these vertical columellæ, together with the two horizontal parallel rods connecting them, represent the complete frontal ring, whilst the third vertical columella is the posterior half of the sagittal ring, the other parts of which are lost. 1. _Prismatium tripodium_, n. sp. (Pl. 93, fig. 22).