Page:Scientific results HMS Challenger vol 18 part 2.djvu/327

Rh Thorax with three stout, curved, widely divergent ribs, prolonged into three cylindro-conical, curved feet of twice the length, which are convex in the proximal half, concave in the distal half. Between every two ribs, beyond the collar stricture, two large ovate meshes, and beyond this two rows of smaller irregular meshes.

Dimensions.—Cephalis 0.04 long, 0.07 broad; thorax 0.05 long, 0.11 broad.

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

3. Sethopilium macropus, n. sp. (Pl. 97, fig. 9).

Shell smooth, with slight collar stricture. Relative length of the two joints = 4 : 7, breadth = 6 : 13. Cephalis hemispherical, with irregular double-contoured pores, about as broad as the bars. Thorax with three very stout, carved, widely divergent ribs, prolonged into three curved, cylindrical, very long feet, which are three to four times as long as the shell, and convex on the outside. Between every two ribs, beyond the collar stricture, two large broad meshes, and beyond this two or three rows of smaller meshes.

Dimensions.—Cephalis 0.04 long, 0.06 broad; thorax 0.07 long, 0.13 broad.

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

Definition.— (vel Dicyrtida triradiata aperta) with three free lateral wings, or solid spines arising from the sides of the thorax. No terminal feet. Cephalis with one or more horns.

The genus Lithomelissa, containing numerous and widely distributed forms, may like Dictyophimus be regarded as one of the most primitive and ancient forms of Dicyrtida. It differs from the latter by the lateral (not terminal) position of the three thoracic feet, and may be derived either from Dictyophimus (by secondary development of intrapedal network towards the mouth, on the inside of the three feet) or directly from Tripospyris by similar formation of thoracic network beyond the collar stricture, outside the base of the three divergent feet and pierced by the latter. As the species of this genus are numerous, it may perhaps be better to divide it into two or three genera: Acromelissa, with a single horn, Micromelissa, with two horns, and Sethomelissa, with three or more horns.

Definition.—Cephalis with a single occipital horn.