_Dimensions._--Shell 0.09 long, 0.07 broad; wings 0.14 long. _Habitat._--Central Pacific, Station 265, depth 2900 fathoms. Genus 501. _Tripilidium_,[118] Haeckel, 1881, Prodromus, p. 428. _Definition._--#Archipilida# (vel Monocyrtida triradiata aperta) without lateral ribs, with three simple or branched terminal feet and an apical horn. The genus _Tripilidium_ differs from _Tripodonium_ in the absence of the three lateral ribs, and may be derived either from this ancestral form, or directly from _Tripospyris_ (by loss of the sagittal ring and constriction); perhaps also from _Lychnocanium_ (by reduction of the cephalis). We distinguish as two subgenera, _Tristylocorys_ (with simple feet) and _Tripodocorys_ (with branched feet). Subgenus 1. _Tristylocorys_, Haeckel. _Definition._--Feet simple, not branched. 1. _Tripilidium cortina_, n. sp. Shell campanulate, rough, about as long as broad. Pores subregular, circular, larger in the middle zone, twice to three times as broad as the bars. Mouth constricted, half as broad as the shell. Apical horn straight and stout, three-sided pyramidal, about as long as the shell. Three feet divergent, slightly S-shaped, curved, somewhat longer the the shell. _Dimensions._--Shell 0.1 diameter, horn 0.09 long, feet 0.13 long. _Habitat._--Central Pacific, Station 272, depth 2600 fathoms. 2. _Tripilidium hemisphæricum_, n. sp. Shell hemispherical, smooth, nearly twice as broad as long. Pores large, with thin bars, irregular, roundish, of very different sizes. Mouth wide open. Apical horn conical, half as long as {1141}the shell. Three feet slender cylindrical, widely divergent, curved, nearly twice as long as the shell. _Dimensions._--Shell 0.08 long, 0.14 broad; horn 0.05 long, feet 0.15 long. _Habitat._--South Pacific, Station 298, depth 2225 fathoms. 3. _Tripilidium lychnocanium_, n. sp. Shell hemispherical, thorny, one and a half times as broad as long. Pores regular, circular, hexagonally framed, as broad as the bars. Mouth slightly constricted. Apical horn stout and short, three-sided pyramidal. Three feet cylindrical, straight, divergent, twice to three times as long as the shell. (Similar to _Lychnocanium favosum_, Pl. 61, fig. 6, perhaps derived from it by reduction of the cephalis.) _Dimensions._--Shell 0.08 long, 0.12 broad; horn 0.02 long, feet 0.2 long. _Habitat._--Central Pacific, Station 265, depth 2900 fathoms. 4. _Tripilidium ovatum_, n. sp. Shell ovate, smooth, one and one half times as long as broad. Pores regular, circular, as broad as the bars. Mouth constricted, half as broad as the shell. Horn short, conical. Three feet conical, curved, convergent, scarcely one-third as long as the shell. _Dimensions._--Shell 0.15 long, 0.11 broad; horn 0.02 long, feet 0.04 long. _Habitat._--South Atlantic, Station 332, depth 2200 fathoms. 5. _Tripilidium clavatum_, n. sp.