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. {1001}Genus 430. _Tympaniscus_,[49] n. gen. _Definition._--#Tympanida# with two bisected horizontal rings, which are connected by three vertical meridional rings (or six columellæ). The genus _Tympaniscus_ and the following _Tympanidium_ differ from all preceding Tympanida in the multiplication of the vertical columellæ, which connect the two horizontal bisected rings. _Tympaniscus_ exhibits six columellæ, which may be regarded as halves of three meridional rings. Only one, however, of these is complete (the sagittal ring, which bisects the mitral and the basal ring); the two other meridional rings (placed in diagonal planes) may be regarded as produced by lateral bifurcation of an incomplete frontal ring; this appears to be already foreshadowed in the preceding _Octotympanum_. 1. _Tympaniscus corona_, n. sp.