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.