Page:Scientific results HMS Challenger vol 18 part 2.djvu/129

Rh

Definition.— with two latticed horizontal rings, connected by numerous descending and diverging columellæ; mitral ring smaller than the basal ring.

The genus Paratympanum and the following nearly allied Lithotympanum form together the small subfamily of Paratympanida, distinguished from the other Tympanida by the fenestration of the two parallel horizontal rings. These rings in Lithotympanum are both equal, whilst in Paratympanum, the basal ring is larger than the mitral ring; therefore the connecting rods or columellæ are in the former parallel and vertical, in the latter divergent and descending. Two opposite ones of these columellæ are parts of the primary sagittal ring, two others are parts of the lateral or frontal ring, whilst the remainder are afterwards intercalated. Paratympanum may be derived from Dystympanium by fenestration of the basal ring.

1. Paratympanum hexastylum, n. sp.

Six divergent columellæ, about as long as the diameter of the hexagonal mitral ring, connect it with the larger hexagonal basal ring. Lattice of the mitral ring hemispherical, with six roundish pores, surrounding a central apical horn. Lattice of the basal ring with one large central pore, surrounded by numerous peripheral pores. Rings and rods smooth, without spines.

Dimensions.—Length of the shell 0.07; breadth of the mitral ring 0.06, of the basal ring 0.09.

Habitat.—Central Pacific, Station 266, depth 2750 fathoms.

2. Paratympanum octostylum, n. sp. (Pl. 94, fig. 14).

Eight divergent columellæ, about as long as the diameter of the octagonal mitral ring, connect it with the larger circular basal ring, and bear at each end a divergent, strong, conical spine. Lattice-plate of both rings slightly convex, with numerous subregular circular pores, smooth, without surface spines.

Dimensions.—Breadth of the mitral ring 0.06, of the basal ring 0.09.

Habitat.—Central Pacific, Station 268, depth 2900 fathoms.

3. Paratympanum decastylum, n. sp.

Ten divergent columellæ, only half as long as the diameter of the decagonal mitral ring, connect it with the larger circular basal ring. Lattice-plate of the mitral ring nearly hemispherical of the basal ring flat, both with numerous irregular roundish pores and scattered spines. Margin of both rings spiny.

Dimensions.—Breadth of the mitral ring 0.08, of the basal ring 0.1.

Habitat.—Central Pacific, Station 271, depth 2425 fathoms.