Page:Scientific results HMS Challenger vol 18 part 2.djvu/133

Rh 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.

Definition.— 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).

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.