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.
2. Dystympanium distephanus, n. sp.
Basal ring hexagonal, with six larger spines at the six corners, and numerous smaller ones between them. Mitral ring half as broad as the basal ring, also with six angular spines. The latter are connected with the former by six divergent curved columellæ, about as long as the diameter of the basal ring. Mitral lattice-plate flat, with irregular roundish pores.
Dimensions.—Diameter of the basal ring 0.1, mitral ring 0.05; length of the columellæ 0.09.
Habitat.—Equatorial Atlantic, Station 347, depth 2250 fathoms.
3. Dystympanium medusa, n. sp.
Basal ring circular, with eight larger and numerous smaller divergent spines. Mitral ring two-thirds as broad as the basal ring, octagonal, with eight conical ascending spines on the eight corners. The latter are connected with the former by eight divergent, curved columellæ, as long as the diameter of the mitral ring. Mitral lattice-plate nearly hemispherical, with nine circular pores (eight peripheral pores around one central pore).
Dimensions.—Diameter of the basal ring 0.12, mitral ring 0.08; length of the columellæ 0.09.
Habitat.—South Atlantic, Station 332, depth 2200 fathoms.
Subfamily 4. Eutympanida, Haeckel.
Definition.—Tympanida with two simple horizontal rings, which are connected by a variable number of vertical rods or columellæ. Therefore the shell exhibits a large simple mitral gate on the upper, and a large simple basal gate on the lower side; between the two a variable number of lateral gates.
