Shell ovate campanulate, one and a half times as long as broad, with regular circular pores, disposed in alternate longitudinal rows. Apical horn three-sided pyramidal, stout, half as long as the shell. In the middle of the shell there arise three wing-like lateral ribs, which are prolonged into three broad, slightly curved, divergent terminal feet. _Dimensions._--Shell 0.12 long, 0.08 broad, horn and feet 0.06 to 0.08. _Habitat._--Fossil in Barbados. 4. _Tripocalpis triserrata_, n. sp. (Pl. 51, fig. 6). Shell ovate, nearly twice as long as broad. The three sides of the shell, between the three large ribs, are provided each with nine delicate parallel, longitudinal, slightly curved ribs, alternating with ten longitudinal rows of regular circular pores about as broad as the bars. Apical horn short and broad, three-sided pyramidal; from the three edges of its base there arise three wing-like serrate main ribs, which are prolonged into three short pyramidal terminal feet. _Dimensions._--Shell 0.12 long, 0.07 broad, horn and feet 0.01. _Habitat._--Central Pacific, Station 271, depth 2425 fathoms. {1137}5. _Tripocalpis cortinaris_, n. sp. (Pl. 97, fig. 3). Shell hemispherical, one and a half times as broad as long, with regular, circular, hexagonally framed pores, Apical horn stout, three-sided pyramidal, with three serrate edges. From the base of the latter arise three strongly dentate and curved lateral ribs, with recurved teeth, and these are prolonged into the serrate convex edge of the three basal feet, which are crescentic and twice as long as the shell. _Dimensions._--Shell 0.07 long, 0.1 broad; horn 0.06 long, feet 0.14 long. _Habitat._--Indian Ocean, Cocos Islands (Rabbe), surface. Genus 497. _Tripodonium_,[114] Haeckel, 1881, Prodromus, p. 428. _Definition._--#Archipilida# (vel Monocyrtida triradiata aperta) with three lateral ribs prolonged into three terminal feet. Apex without horn. The genus _Tripodonium_ differs from the preceding _Tripocalpis_, its probable ancestral form, only in the loss of the apical horn (already very small in some forms of the latter). It bears therefore to the latter the same relation that _Sethopilium_ does to _Dictyophimus_. 1. _Tripodonium campanulatum_, n. sp. Shell campanulate, somewhat broader than high, with regular circular pores, twice as broad as the bars. Three broad lateral wings, lamellar, triangular, are prolonged over the peristome into three short triangular lamellar feet one-fourth as long as the shell. _Dimensions._--Shell 0.08 long, 0.09 broad; feet 0.02 long. _Habitat._--Central Pacific, Station 272, depth 2600 fathoms. 2. _Tripodonium ovatum_, n. sp. Shell ovate, one and one-third times as long as broad, with numerous and small circular pores, scarcely as broad as the bars. Three prominent longitudinal ribs, arising in the lower half of the shell-wall, are prolonged into three divergent conical curved feet, about half as long as the shell.