Page:Scientific results HMS Challenger vol 18 part 2.djvu/483

Rh

Definition.— (vel Tricyrtida triradiata clausa) with three lateral wings on the sides of the inversely conical abdomen, which bears no vertical terminal horn on the basal apex.

The genus Rhopalocanium, and the closely allied Rhopalatractus which follows, differ from the preceding Theopera, their probable ancestral form, in the origin of the three lateral wings from the inversely conical abdomen, the thorax bearing no wings. The upper part of the wing bases, which arose formerly from the thorax, is here reduced and lost.

1. Rhopalocanium lasanum, n. sp. (Pl. 67, fig. 10).

Shell subovate, with two distinct strictures. Length of the three joints = 1 : 2 : 3, breadth = 1 : 3 : 4. Cephalis hemispherical, with a conical, dimpled horn of twice the length. Thorax subconical, abdomen campanulate, both with irregular, circular pores. Three wings cylindrical, divergent, about as long as the shell, striated, with an ovate, dimpled knob at the distal end, their broad triangular base being attached to the upper half of the abdomen. (Form rather variable.)

Dimensions.—Length of the three joints, a 0.02, b 0.07, c 0.09; breadth, a 0.03, b 0.09, c 0.12.

Habitat.—Central Pacific, Stations 263 to 268, depth 2650 to 3000 fathoms.

2. Rhopalocanium ornatum, Ehrenberg.

Rhopalocanium ornatum, Ehrenberg, 1854, Mikrogeol., Taf. xxxvi. fig. 9; Abhandl. d. k. Akad. d. Wiss. Berlin, 1875, p. 82, Taf. xvii. fig. 8.

Shell nearly spindle-shaped, with two distinct strictures. Length of the three joints = 1 : 3 : 6, breadth = 1 : 4 : 3. Cephalis hemispherical, with a stout, cylindrical horn of twice the length, bearing a dimpled cone. Thorax ovate, campanulate. Abdomen inversely conical. Pores subregular, circular, twice as large in the thorax as in the abdomen. Three wings compressed, slender, divergent, slightly longer than the abdomen, bearing an ovate, dimpled cone at their distal end, with the broad, triangular, striated base attached to the upper third of the abdomen.

Dimensions.—Length of the three joints, a 0.02, b 0.06, c 0.12; breadth, a 0.025, b 0.08, c 0.07.

Habitat.—Fossil in Barbados.

3. Rhopalocanium cortinium, n. sp.

Shell slender, ovate, nearly of the same form as, and with similar fenestration to that of Theopera cortina, (Pl. 67, fig. 8). It differs from the latter mainly by the absence of the three thoracic ribs, characteristic of Theopera. Length of the three joints = 1 : 2 : 6, breadth = 1 : 4 : 4. The