Page:Scientific results HMS Challenger vol 18 part 2.djvu/374

1250 The genus Sethamphora differs from the preceding Sethophormis, and from the next following ribbed Sethophormida, mainly in the small, more or less constricted mouth; therefore the thorax is more or less ovate, and its ribs curved, with the convexity external. Sethophormis may be divided into two different subgenera, Dictyoprora and Cryptocephalus; in the former the cephalis is perfectly free and well developed, in the latter it is more or less hidden in the uppermost part of the thorax (like Carpocanium).

Definition.—Cephalis large, perfectly free, not hidden in the thorax.

1. Sethamphora hexapleura, n. sp.

Shell ovate, smooth, with distinct collar stricture. Length of the two joints = 3 : 11, breadth = 5 : 10. Cephalis large, free, hemispherical, with small circular pores. Thorax truncate, ovate, with six strong curved ribs, and six large triangular meshes between them in the upper half; five smaller pores between every two ribs in the lower half (in three tiers); therefore, together, thirty-six thoracic meshes. Mouth strongly constricted, smooth, only one-third as broad as the thorax.

Dimensions.—Cephalis 0.03 long, 0.05 broad; thorax, 0.11 long, 0.1 broad.

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

2. Sethamphora enneapleura, n. sp.

Shell subovate, smooth, with slight collar stricture. Length of the two joints = 3 : 9, breadth = 4 : 8. Cephalis large, hemispherical, free, with small circular pores. Thorax inflated, ovate, with nine strong curved ribs, which are connected by five transverse circular rings at equal distances. Forty-five rounded, squarish, large meshes between them, in five transverse and nine longitudinal rows. Mouth constricted, smooth, half as broad as the thorax.

Dimensions.—Cephalis 0.03 long, 0.04 broad; thorax 0.09 long, 0.08 broad.

Habitat.—South Atlantic, Station 332, depth 2200 fathoms.

3. Sethamphora dodecapleura, n. sp.

Shell urceolate, smooth, with sharp collar stricture. Length of the two joints = 3 : 9, breadth = 5 : 9. Cephalis flat, cap-shaped, free, with twelve ribs, and very small pores between them. Thorax thick-walled, inflated, with subcylindrical cavity, and twelve prominent sharp ribs, alternating with twelve longitudinal rows of very small circular pores (eight pores in each row). Mouth constricted, half as broad as the thorax.

Dimensions.—Cephalis 0.03 long, 0.05 broad; thorax 0.09 long, 0.09 broad.

Habitat.—Western Tropical Pacific, Station 225, depth 4475 fathoms.