The genus _Lithotympanum_ has the form of a cylindrical drum, the two fundamental faces of which (the mitral and basal gates) are closed by lattice-work. Both horizontal rings are equal, whilst in the preceding _Paratympanum_ the basal gate is larger than the mitral. It may be derived from the latter by equalisation of these two rings, or directly from _Dystympanium_ by fenestration of the basal ring. 1. _Lithotympanum tuberosum_, n. sp. (Pl. 83, fig. 1). Both lattice-plates slightly convex; the mitral plate with six larger marginal pores and one central pore separated by numerous smaller pores; the basal plate with seven large pores only (one central and six peripheral). Surface and margin of the plates uneven, with roundish prominent dimpled tubercles. Both rings connected by ten to twelve simple cylindrical vertical columellæ. _Dimensions._--Length of the shell 0.13, breadth 0.1. _Habitat._--Western Tropical Pacific, Station 225, depth 4475 fathoms. 2. _Lithotympanum spinosum_, n. sp. Both lattice-plates flat, with numerous irregular roundish pores of variable size. Surface thorny. Margin with numerous irregular spines, directed upwards on the mitral, downwards on the basal plate. Both rings connected by eight simple cylindrical vertical columellæ. _Dimensions._--Length of the shell 0.12, breadth 0.08. _Habitat._--Western Tropical Pacific, Station 224, depth 1850 fathoms. Subfamily 3. DYSTYMPANIDA, Haeckel. _Definition._--#Tympanida# with fenestrated mitral ring and simple open basal ring; both horizontal rings connected by numerous vertical or divergent columellæ. Genus 434. _Dystympanium_,[53] n. gen. _Definition._--#Tympanida# with two horizontal rings, connected by numerous (six to eight or more) divergent columellæ. The upper mitral ring is smaller and latticed, the lower basal ring larger and open. {1007}The genus _Dystympanium_, representing a peculiar subfamily, differs from all other Tympanida in the different shape of the two parallel horizontal rings. The upper or mitral ring is closed by a lattice-plate (as in _Paratympanum_), the lower or basal ring is simple and open (as in _Eutympanium_). _Dystympanium_ may therefore be regarded as an intermediate form between these two genera, and may perhaps be derived from _Paratympanum_ by loss of the lower lattice-plate. 1. _Dystympanium dictyocha_, n. sp. (Pl. 94, figs. 15, 16). Basal ring circular, with eighteen to twenty short conical divergent spines. Mitral ring about two-thirds as broad, hexagonal, with six ascending spines on the six corners. The latter are connected with the former by six simple, straight, obliquely descending columellæ, about as long as the diameter of the mitral ring. Mitral lattice-plate slightly convex, with seven roundish pores (six peripheral pores around a central pore). _Dimensions._--Diameter of the basal ring 0.09, mitral ring 0.06; length of the columellæ 0.05. _Habitat._--Tropical Atlantic, Station 338, depth 1990 fathoms.