Page:Scientific results HMS Challenger vol 18 part 2.djvu/544

1420 of pores, which are twice to four times as large as those of the thorax. Mouth truncated, with thickened margin, two-thirds as broad as the abdomen.

Dimensions.—Length of the three joints, a 0.04, b 0.12, c 0.16; breadth, a 0.04, b 0.16, c 0.2.

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

Definition.— (vel Tricyrtida eradiata aperta), with ovate abdomen, including an internal vertical axial rod, which bears three divergent radial spines or verticils of three branches, and is prolonged into an apical horn.

The genus Axocorys contains only a single but very remarkable species, and may, perhaps, represent a peculiar group, which has closer relations to the than to the other Tricyrtida. The pyriform three-jointed shell has neither lateral nor terminal free appendages, but possesses a very large apical horn, and an inner prolongation of this, an axial rod, which bears some triradiate verticils of branched spines. The original ancestral forms of this remarkable genus are probably Plagoniscus and Plectaniscus (pp. 912 and 924).

1. Axocorys macroceros, n. sp. (Pl. 68, figs. 1, 1a).

Shell pear-shaped, with two distinct strictures. Length of the three joints = 2 : 3 : 16, breadth = 3 : 6 : 16. Cephalis hemispherical, with few small pores. From its apex arises vertically a very long three-sided prismatic horn with denticulate edges, three to four times as long as the shell. An inner thin prolongation of the horn descends vertically nearly to the mouth, and bears on its basal end three verticils of branched spines, each verticil with three divergent forked spines (fig. 1a). Pores regular, circular, in the subspherical abdomen three times as large as in the hemispherical thorax. The constricted mouth is scarcely broader than the thorax.

Dimensions.—Length of the three joints, a 0.02, b 0.03, c 0.16; breadth a 0.03, b 0.06, c 0.16.

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

Definition.— (vel Tricyrtida eradiata aperta), with ovate abdomen, broader than the constricted mouth. Cephalis with two divergent horns, or with a bunch of horns.