Page:Scientific results HMS Challenger vol 18 part 2.djvu/851

Rh acute corners jutting out near the aboral hinge, one corner in the keel of the dorsal, the outer in the keel of the ventral valve. Borders of the two boat-shaped valves smooth in 0.3 of the oral part, and in 0.1 of the aboral part, strongly dentated in the remaining 0.6 middle part; about thirty-five teeth in one side of each valve, larger on both ends than in the middle. In the half frontal perimeter of the shell sixty to sixty-five pores, in the half sagittal perimeter eighty to eighty-five, in the half equator of the shell forty to fifty pores.

Dimensions.—Length of the shell 0.55 to 0.65, height 0.5 to 0.55, breadth 0.2 to 0.22.

Habitat.—North Pacific, Stations 243 and 244, depth 2800 to 2900 fathoms.

7. Conchopsis navicula, n. sp. (Pl. 125, figs. 4-6).

Shell pear-shaped, compressed on both sides, in the sagittal periphery keeled. Proportion of the longitudinal diameter to the sagittal and lateral = 4 : 3 : 2. Its sagittal perimeter nearly ovate. Borders of the two boat-shaped valves smooth in 0.3 of the oral, and 0.15 of the aboral part, strongly dentated in the remaining 0.55 middle part; teeth conical, of nearly equal size. In the half frontal perimeter of the shell (along one border of each valve) forty to forty-five pores, in the half sagittal fifty-four to fifty-six pores, in the half equator thirty-two to thirty-six pores. Each pore is surrounded by a hexagonal frame, and pierces the shell in an oblique direction, dilated in the middle part (figs. 5, 6). Shell very thick-walled, several longitudinal crests on both sides of the keel of each valve. Hinge very strong, usually with a broad ligament between the two unequal aboral lips of the hinge.

Dimensions.—Length of the shell 0.8, height 0.6, breadth 0.4.

Habitat.—South Pacific, Station 293, depth 2025 fathoms.

Definition.— with dentate lateral margins and a sharp sagittal keel of the compressed valves, and with two caudal horns on the hinge (a dorsal and a ventral).

The genus Conchoceras has the same lenticular keeled and laterally compressed shell as the preceding ancestral genus Conchopsis, but is distinguished from it by the development of two large caudal horns on the aboral hinge. It bears therefore the same relation to the latter as Conchidium does to Conchellium.

1. Conchoceras caudatum, n. sp. (Pl. 124, fig. 15).

Shell lenticular, slightly compressed; proportion of the longitudinal diameter to the sagittal and lateral = 6 : 5 : 4; sagittal and cinctural perimeter ovate, frontal perimeter elliptical. Free