3. Challengeria sigmodon, n. sp.
Shell circular, nearly spherical, slightly compressed. Peristome with a single simple tooth, which is S-shaped, more or less curved, and about one-third as long as the shell.
Dimensions.—Diameter of the shell 0.3 to 0.4, length of the tooth 0.1 to 0.14.
Habitat.—North Pacific, Station 231, depth 2250 fathoms.
4. Challengeria tritonis, n. sp. (Pl. 99, fig. 5).
Shell ovate, slightly compressed. Peristome with a single simple tooth, which is vertical, straight, slenderly conical, and about half as long as the shell.
Dimensions.—Diameter of the shell 0.15 to 0.2, length of the tooth 0.1 to 0.12.
Habitat.—North Atlantic, Færöe Channel, Gulf Stream, in great numbers, in depths from 100 to 600 fathoms. (Expedition of H.M.S. "Triton," 1882), John Murray.
5. Challengeria pyramidalis, n. sp.
Shell slenderly ovate, one and a half times as long as broad, slightly compressed. Peristome with a single, simple, large tooth, which is three-sided pyramidal, vertical, straight, and longer than half the shell.
Dimensions.—Length of the shell 0.6 to 0.8, breadth 0.4 to 0.5; length of the tooth 0.4 to 0.5.
Habitat.—South Atlantic, Station 318, depth 2040 fathoms.
6. Challengeria cyrtodon, n. sp.
Shell ovate, lenticular, strongly compressed. Peristome with a single simple tooth, which is crescent-shaped, curved over the mouth, and about half as long as the shell.
Dimensions.—Length of the shell 0.4 to 0.5, breadth 0.3 to 0.4; length of the tooth 0.2 to 0.25.
Habitat.—South Pacific, Station 289, depth 2550 fathoms.
Subgenus 2. Challengeretta, Haeckel.
Definition.—Peristome with a forked or bifid tooth, or with two parallel lateral teeth (Challengeria diodontes).
7. Challengeria sloggettii, John Murray (Pl. 99, fig. 4).
Shell triangular, strongly compressed, with three rounded corners, or nearly semicircular; its aboral margin straight, horizontal, longer than the convex, dorsal, and ventral margins. Peristome
