Mitral gates of the same size as the basal gates; upper and lower halves of the meridional rings of equal size. Rings armed with numerous stout, thorny, simple and forked spines; four very large branched spines in the equatorial plane, nearly horizontal, two on the poles of the sagittal, two on the poles of the transverse axis, the latter curved downwards.
Dimensions.—Length of the shell 0.1, breadth 0.2.
Habitat.—Central Pacific, Station 271, depth 2425 fathoms.
3. Octotympanum arborescens, n. sp.
Mitral gates smaller than the basal gates, the lower halves of the lateral rings being broader than the upper halves. Rings very spiny, armed with numerous branched spines; ten very large arborescent spines on the lateral rings; two on the poles of the transverse axis, eight on the prominent corners of the diagonal axes (four upper and four lower); sagittal ring thorny.
Dimensions.—Length of the shell 0.8, breadth 0.16.
Habitat.—Central Pacific, Station 274, depth 2750 fathoms.
4. Octotympanum cervicorne, n. sp. (Pl. [94], fig. 2).
Mitral and basal gates of equal size, the lower and upper halves of the two crossed lateral rings being nearly equal. Rings armed with numerous branched spines; two very large spines, similar to the antlers of a deer, on the poles of the lateral axis; their branches about half as large as the whole shell. Sagittal ring nearly smooth, with few small thorns on the poles of the main axis.
Dimensions.—Length of the shell 0.16, breadth 0.2.