Basal ring of about the same size as the mitral ring, with six large conical descending feet; these are prolongations of the six columellæ, nearly of the same length, slightly divergent, and bear at the base a strong horizontal spur. The six columellæ are strongly curved, spiny, and bear above small arches, which are united with the apical horn of the sagittal ring. _Dimensions._--Breadth of the shell 0.09, height 0.05; total height 0.09. _Habitat._--Fossil in Barbados. 7. _Tympaniscus dodecaster_, n. sp. Basal ring of the same size as the mitral ring, with six very large and six alternating smaller, slightly divergent, curved feet, about as long an the shell; the larger are the basal prolongations of the six curved, thorny columellæ. Mitral ring spiny, not overgrown with arches, as in the similar preceding species. _Dimensions._--Length of the shell 0.08, breadth 0.12. _Habitat._--Tropical Atlantic, Station 347, depth 2250 fathoms. {1003}Genus 431. _Tympanidium_,[50] Haeckel, 1881, Prodromus, p. 447. _Definition._--#Tympanida# with two bisected horizontal rings, which are connected by four vertical meridional rings (or eight columellæ). The genus _Tympanidium_ differs from all other Protympanida in the development of four vertical rings, placed in four different meridional planes. Only one of these is complete, the primary sagittal ring; the three others are incomplete, inserted on the two horizontal rings, which are formed by the paired mitral and basal branches of the sagittal ring; one of these three lies in the frontal plane, the two others in diagonal meridional planes (between the frontal and sagittal). The shell therefore exhibits twelve large gates, four of which are horizontal (the two superior mitral and the two inferior basal gates); the eight others are vertical, separated by the eight columellæ, or the halves of the four meridional rings. In the subgenus _Tympanomma_ the number of gates amounts to sixteen, the four lateral gates being bisected by an incomplete equatorial ring. The genus _Tympanidium_ may be derived from _Acrocubus_ by development of the two diagonal rings (between the frontal and sagittal). Subgenus 1. _Tympanura_, Haeckel. _Definition._--Shell with twelve gates; the four lateral gates simple. 1. _Tympanidium foliosum_, n. sp. (Pl. 94, fig. 1). Shell with twelve gates; the four lateral gates simple. Basal gates of the shell larger than the mitral gates. Sagittal ring ovate, much smaller than the three other meridional rings, which are armed with numerous large elegant spines, bearing a lanceolate leaf on a thin pedicle. The sagittal ring bears a bunch of similar spines only at the apex, and on both poles of the sagittal axis (in the equator) a single forked spine, with two thorny branches. _Dimensions._--Length of the shell 0.09 (with spines 0.25), breadth 0.15. _Habitat._--South Pacific, Station 300, depth 1375 fathoms. 2. _Tympanidium spinosum_, n. sp.