Page:Scientific results HMS Challenger vol 18 part 2.djvu/845

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Definition.— with the lateral margins of the valves dentate, without sagittal keel and apical horns, but with two caudal horns on the hinge (a dorsal and a ventral).

The genus Conchidium is the most common form of all Concharida, and some of its species occur in great numbers in the tropical zone of the Pacific and the Atlantic, on the surface as well as at various depths. It differs from the preceding Conchellium, its ancestral form, in the development of two caudal horns, or two strong pyramidal spines which arise from the posterior end of the valves; the dorsal horn usually is smaller than the ventral.

1. Conchidium terebratula, n. sp. (Pl. 124, figs. 1, 2).

Shell subspherical, smooth; both valves of nearly equal size and form, hemispherical. The three dimensive axes of the body are almost equal. Margins of the valves dentate in nearly the whole periphery; on each side of one valve eleven or twelve strong conical teeth, all of the same size. Aboral hinge with two short and stout four-sided pyramidal horns of equal length. Mouth with two equal short lips. Pores of the shell subregular, circular, three to four times as broad as the bars, in the dorsal valve twice as large as in the ventral valve.

Dimensions.—Diameter of the shell 0.24 to 0.28.

Habitat.—Central Pacific, Stations 270 to 274, surface, and at various depths.

2. Conchidium thecidium, n. sp. (Pl. 124, fig. 6).

Shell subspherical, slightly compressed on both sides. Dorsal valve somewhat smaller than the ventral, of similar form. Principal axis of the shell somewhat longer than the sagittal, and this longer than the frontal axis. Margins of the valves smooth in the oral quarter, strongly dentate in the remainder; on each side of one valve eight or nine very large triangular teeth, half as long as the height of the valve. Aboral hinge with two unequal, stout, four-sided pyramidal horns; the dorsal horn half as long at the ventral. Mouth with two unequal lips, the upper shorter than the lower. Pores of the shell subregular, hexagonal, three to four times as broad as the bars. The figured specimen, well preserved, contained in the central capsule two nuclei, one in the dorsal, the other in the ventral half.

Dimensions.—Length of the shell 0.28 to 0.33, height 0.27 to 0.3, breadth 0.22 to 0.24.

Habitat.—South Pacific, Station 292, depth 1600 fathoms.