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 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.