Page:Scientific results HMS Challenger vol 18 part 2.djvu/119

Rh 9. Acrocubus amphithectus, n. sp. (Pl. 93, fig. 17).

Mitral ring smaller than the basal, both rings elliptical, with curved rods. Four columellæ thorny, also curved, divergent downwards, prolonged upwards into four thorny convergent horns, downwards into four larger branched divergent feet. The two sagittal horns and feet (anterior and posterior) are smaller than the two lateral (right and left); therefore the fundamental form of this species is distinctly amphithect (as in the Ctenophora).

Dimensions.—Breadth of the cube 0.12; total height 0.2.

Habitat.—South Pacific, Station 289, depth 2550 fathoms.

Definition.— with two bisected horizontal rings, connected by four vertical columellæ. In the frontal plane two galear bows project upwards, and two thoracal bows downwards.

The genus Toxarium differs from its ancestral form Acrocubus in the development of two pairs of peculiar bows, placed in the lateral or frontal plane; the upper pair projects above the mitral gates, and may be called the "galear bows," the lower pair projects below the basal gates of the cubical shell, and may be called the "thoracal bows." Both pairs are produced originally by the development of two forked vertical spines, arising in the frontal plane on both poles of the principal axis from the sagittal ring (compare Protympanium amphipodium, Pl. 93, fig. 14). The apical fork-branches become connected with the lateral corners of the mitral gates, the basal branches with those of the basal gates. By development of lattice-work between the bows and the rings some (Amphispyris, &c.) may have arisen. The important genus Toxarium may be divided into three subgenera. In Toxellium all the bows are simple, in Toxonium, all are forked; in Toxidium, the galear bows are simple, the thoracal bows forked.

Definition.—Galear and thoracal bows simple.

1. Toxarium circospyris, n. sp. (Pl. 88, fig. 1).

Galear and thoracal bows simple, of about the same size as the frontal bows between them; all bows armed with scattered simple conical spines. Sagittal constriction deep, half as long as the whole shell. Lateral outlines of the shell nearly parallel.

Dimensions.—Length of the shell 0.16, breadth 0.13.

Habitat.—Central Pacific, Stations 265 to 274, depth 2350 to 2925 fathoms.