Habitat.—North Pacific, Stations 250 to 253, depth 2740 to 3125 fathoms.

2. Conchellium hippopus, n. sp.

Shell nearly spherical, in the lateral perimeter (along the girdle-fissure) somewhat constricted. Borders of the two hemispherical valves semicircular, smooth in 0.3 of the oral, and 0.2 of the aboral part, dentated only in the remaining 0.5 middle part; on one side of each valve seven to eight very strong and long teeth, increasing in size towards the mouth, the longest (foremost) teeth nearly one-third as long as the shell-radius. In the half frontal perimeter of the shell (along one border of each valve) twenty-four to twenty-eight pores, in the half sagittal perimeter thirty-two to thirty-six, in the half equator twelve to fourteen. Pores circular, twice to three times as broad as the bars, smaller along the fissure.

Dimensions.—Length of the shell 0.06, height 0.065, breadth 0.055.

Habitat.—Central area of the Tropical Pacific, Station 274, depth 2750 fathoms.

Genus 723. Conchidium,[[339]] Haeckel, 1879, Sitzungsb. med.-nat. Gesellsch. Jena, Dec. 12, p. 6.

Definition.—Concharida 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.