Mitral and apical ring of equal size, elliptical, smooth, somewhat smaller than the connecting thorny sagittal ring.
Dimensions.—Diameter of the two horizontal rings 0.12, of the sagittal ring 0.15.
Habitat.—Central Pacific, Stations 263 to 274, at various depths.
2. Protympanium amphipodium, n. sp. (Pl. [93], fig. 14).
Mitral and apical rings of equal size, thin, elliptical, smooth, larger than the broad connecting sagittal ring. The latter bears on the apical and on the basal pole two divergent straight spines (lying in the frontal plane), the two apical (or horns) somewhat smaller and not so spiny as the two basal spines (or feet). In the frontal plane there arise from the two horizontal rings on each side two small opposite spines (remnants or beginnings of the lateral frontal ring?).
Dimensions.—Diameter of the two horizontal rings 0.11, of the sagittal ring 0.07.
Habitat.—Central Pacific, Station 271, depth 2425 fathoms.
3. Protympanium trissocircus, n. sp.
Mitral ring smaller than the basal ring; both rings circular, rough. Sagittal ring in size between the two former. All three rings covered with very small thorns.
Dimensions.—Diameter of the mitral ring 0.08, of the sagittal ring 0.1, of the basal ring 0.12.