Page:Scientific results HMS Challenger vol 18 part 2.djvu/484

1360 three wings arise by a broad, triangular, striated base from the upper half of the abdomen, below the lumbar stricture. The cephalic horn and the three divergent wings are shorter, and at the distal end not so much thickened as in Theopera cortina.

Dimensions.—Length of the three joints, a 0.025, b 0.045, c 0.12; breadth, a 0.03, b 0.07, c 0.08.

Habitat.—Tropical Atlantic, Station 347, depth 2250 fathoms.

4. Rhopalocanium pythia, n. sp.

Rhopalocanium ornatum, Bury, 1862, Polycystins of Barbados, pl. vi. fig. 1.

Shell nearly ovate, with two deep strictures. Length of the three joints = 1 : 4 : 8, breadth = 1 : 4 : 4. Cephalis subspherical, with a conical horn of three times the length. Thorax inflated, abdomen inversely campanulate and prolonged into a short, conical, latticed tube. Pores subregular, circular. Three wings slender, as long as the abdomen, arising by a broader base from the uppermost part of the abdomen, their thickened end being curved inwards.

Dimensions.—Length of the three joints, a 0.02, b 0.07, c 0.15; breadth, a 0.03, b 0.08, c 0.07.

Habitat.—Fossil in Barbados.

5. Rhopalocanium delphicum, n. sp. (Pl. 67, fig. 9).

Shell nearly spindle-shaped, with two deep strictures. Length of the three joints = 1 : 2 : 4, breadth = 1 : 3 : 3. Cephalis hemispherical, with a slender, conical, curved horn, as long as the hemispherical thorax. Abdomen inversely campanulate, prolonged into a short, conical, latticed tube. Pores subregular, circular. Three wings arising by a narrow base from the uppermost part of the abdomen, slender, cylindrical, S-shaped, curved, divergent, with a thick scaly appendix like a fir-cone at the distal end.

Dimensions.—Length of the three joints, a 0.025, b 0.07, c 0.14; breadth, a 0.03, b 0.09, c 0.08.

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

Definition.— (vel Tricyrtida triradiata clausa) with three lateral wings on the sides of the inversely conical abdomen, which bears a vertical terminal horn on the basal apex.

The genus Rhopalatractus, one of the most remarkable forms of Tricyrtida, differs from the preceding Rhopalocanium, its ancestral form, in the production of a vertical, basal horn, which descends from the basal apex of the inversely conical abdomen, and is opposed to the upper apical horn of the cephalis. The shell becomes here, therefore, exquisitely spindle-shaped.