only two of the former are visible, and represent the figure of a vertical 8; seen from the sagittal poles, only two of the latter are visible, in the form of a larger horizontal 8. Octotympanum may be derived from Acrocubus by reduction of the lateral frontal ring to a knot (or cross-point), and by maximal extension of the two parallel horizontal rings, which become curved downwards laterally and touch at the lateral poles.
1. Octotympanum octospinum, n. sp.
Mitral gates smaller than the basal gates; upper halves of the two lateral meridional rings smaller than the lower halves. Rings with few scattered thorns; four larger simple spines, nearly horizontal in the equatorial plane; two on the poles of the sagittal axis, two on the poles of the lateral axis. Four other large spines, nearly vertical at the four lowermost basal points of the subfrontal rings.
Dimensions.—Length of the shell 0.09, breadth 0.12.
Habitat.—Central Pacific, Station 273, depth 2350 fathoms.
2. Octotympanum octonarium, n. sp. (Pl. 94, fig. 3).
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.
Habitat.—Central Pacific, Station 272, depth 2600 fathoms.
