Page:Scientific results HMS Challenger vol 18 part 2.djvu/486

1362 subspherical. Abdomen subovate, prolonged into a pyramidal, axial, basal spine, half as long as the thorax. Pores subregular, circular. From the upper half of the abdomen arise three stout, lateral, diverged wings of about the same length, which are curved inwards and fenestrated both at the broad triangular base and at the thickened, three-edged distal end.

Dimensions.—Length of the three joints, a 0.02, b 0.06, c 0.12; breadth, a 0.03, b 0.08, c 0.08.

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

Definition.— (vel Tricyrtida triradiata clausa) with a three-sided pyramidal abdomen, the triangular base of which is prolonged at the three corners into three terminal feet.

The genus Lithochytris differs from all the other Theoperida in the absence of lateral wings and the possession of three divergent, terminal feet, which arise from the three corners of the three-sided pyramidal abdomen. It repeats, therefore, among the Tricyrtida, that characteristic formation which Sethochytris and Tetrahedrina represent among the Dicyrtida. Some species may be derived from Pterocanium, other species from Podocyrtis, by the development of a lattice-plate closing the terminal mouth.

Three feet of the abdomen solid, not fenestrated, representing external apophyses of the triangular shell-base.

1. Lithochytris cortina, n. sp. (Pl. 67, fig. 12).

Shell three-sided pyramidal, without external strictures, but with two internal girdles. Length of the three joints = 2 : 2 : 5, breadth = 3 : 4 : 6. Cephalis large, truncate-pyramidal, with nine deep divergent ribs (and nine longitudinal rows of pores between them), and with a stout pyramidal horn of half the length. The three prominent edges of the thorax and abdomen are prolonged over the base of the pyramid into three very stout, solid, subovate feet, about as long as the thorax.

Dimensions.—Length of the three joints, a 0.04, b 0.04, c 0.1; breadth, a 0.06, b 0.08, a 0.12.

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

2. Lithochytris pyriformis, n. sp. (Pl. 67, fig. 13).

Shell pyramidal, nearly pear-shaped, with two distinct strictures. Length of the three joints = 1 : 1 : 3, breadth = 1 : 2 : 3. Cephalis large, pear-shaped, with a short pyramidal horn of half the