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).
Challengeria harstoni, John Murray, 1879 (partim), in litteris, Narr. Chall. Exp., vol. i. p. 226, pl. A, fig. 14.
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 with two parallel, triangular pointed teeth, which are obliquely ascending, lamellar, with inflated ovate base, and about as long as the shell-radius.
Dimensions.—Diameter of the shell 0.2 to 0.3, length of the teeth 0.1 to 0.15.