Subgenus 1. Triostephus, Haeckel, 1881, Prodromus, p. 445.
Definition.—Sagittal and frontal ring of different size and form.
1. Tristephanium dimensivum, n. sp. (Pl. [93], fig. 9).
Sagittal ring ovate, with a large thorny apical horn, smaller than the two other rings. Frontal ring violin-shaped, with a deep sagittal constriction. Basal ring also violin-shaped, in the sagittal axis constricted. Its four gates (two jugular and two cardinal) of nearly equal size. All three rings angular, armed with scattered, stout, thorny spines, which are larger in the basal half.
Dimensions.—Height of the frontal ring 0.08, breadth 0.12.
Habitat.—North Pacific, Station 241, depth 2300 fathoms.
2. Tristephanium octopyle, n. sp. (Pl. [93], fig. 8).
Sagittal ring ovate, nearly smooth, smaller than the two other rings. Frontal ring kidney-shaped, with a deep basal constriction. Basal ring violin-shaped. The jugular gates about half as large as the cardinal gates. Rods of the rings cylindrical, with few scattered thorns.
Dimensions.—Height of the frontal ring 0.09, breadth 0.14.
Habitat.—Central Pacific, Station 271, depth 2425 fathoms.