Shell rough, ovate, very thick-walled, with subregular, circular pores of somewhat variable size. The diameter of the internal, simple, subspherical cavity is five to six times as great as the thickness of the wall. Mouth tubular, cylindrical, very narrow, only one-sixth as broad as the shell. _Dimensions._--Shell 0.16 long, 0.11 broad; mouth 0.02 broad. _Habitat._--Western Tropical Pacific, Station 225, depth 4475 fathoms. Genus 530. _Mitrocalpis_,[147] Haeckel, 1881, Prodromus, p. 427. _Definition._--#Archicorida# with a double ovate or ellipsoidal shell, composed of two parallel lattice-plates, which are connected by numerous beams. Apex without horn. The genus _Mitrocalpis_ and the following closely allied _Spongocyrtis_ differ from the four preceding genera of Archicorida in the complicated structure of the shell-wall, which in the former is double, in the latter spongy. The mouth is in both genera constricted, narrow, and the apex without a horn. _Mitrocalpis_ has been derived probably from _Cyrtocalpis_ in the same way as _Peripyramis_ from _Bathropyramis_, by development of spines on the surface of the shell, which at equal distances from the latter become connected by branches, thus forming an outer envelope. 1. _Mitrocalpis palliata_, n. sp. Shell ellipsoidal, large, one and a third times as long as broad. Inner shell very thin-walled, with irregular, polygonal pores of very variable size and form. From its nodal points there arise numerous bristle-shaped radial beams, about as long as the diameter of the mouth, which are connected at equal distances from the surface by delicate threads, forming the outer enveloping shell. The surface of the latter is smooth. The thin bars of the inner shell are twice to four times as broad as those of the outer. Mouth about one-seventh as broad as the shell, with a short tubular peristome. _Dimensions._--Shell 0.35 long, 0.27 broad; mouth 0.04, distance of both shells 0.037. _Habitat._--Central Pacific, Station 271, surface. Genus 531. _Spongocyrtis_,[148] Dunikowski. _Spongocyrtis_, Dunikowski, 1882, Denkschr. d. k. Akad. d. Wiss. Wien, vol. xlv. p. 31. _Definition._--#Archicorida# with a spongy ovate shell, the wall of which is composed of irregular spongy wickerwork. Apex without horn. The genus _Spongocyrtis_ differs from most other Monocyrtida in the spongy structure of the shell, which is found in only very few other genera of #Cyrtoidea#, e.g., in _Spongopyramis_ and _Spongomelissa_. It has been derived either from the similar {1189}_Cyrtocalpis_ by development of irregular spongy wickerwork on the surface of the simple lattice-shell; or from _Arachnocalpis_ (Pl. 98, fig. 13) by loss of the peristome. 1. _Spongocyrtis montis ovis_, Dunikowski. _Spongocyrtis montis ovis_, Dunikowski, 1882, Denkschr. d. k. Akad. d. Wiss. Wien, vol. xlv. p. 31, Taf. vi. figs. 67, 68.