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.
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.