Page:Scientific results HMS Challenger vol 18 part 2.djvu/521

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Definition.— (vel Tricyrtida eradiata aperta) with discoidal or flatly expanded abdomen. Cephalis with a single apical horn, or with two divergent horns.

The genus Theocalyptra, and the following closely allied genus Cecryphalium, differ from all the other Theocorida in the flattened form of the cap-shaped or nearly discoidal shell. The abdomen specially is quite flat, often horizontally expanded, and like the marginal brim of a flat hat. They may be derived from the Dicyrtida Sethocephalus by the development of an abdomen, or from Corocalyptra by loss of the collar wings. The network is usually very delicate, as in the latter. The cephalis of Theocalyptra bears one or two horns.

1. Theocalyptra veneris, n. sp.

Shell flatly campanulate, very delicate and fragile, very similar to Eucecryphalus agnesæ (Pl. 59, fig. 3), but without collar spines. Length of the three joints = 1 : 4 : 2, breadth = 1 : 7 : 9. Cephalis hemispherical, with a single bristle-shaped horn of twice the length. Thorax subconical, separated from the flatter abdomen by a deep stricture, with thin bars, and large, subregular, hexagonal meshes, increasing in size towards the smooth mouth-edge.

Dimensions.—Length of the three joints, a 0.02, b 0.07, c 0.04; breadth, a 0.025, b 0.15, c 0.1.

Habitat.—Cosmopolitan; Atlantic, Indian, Pacific, many stations, surface.

2. Theocalyptra orci, Haeckel.

Halicalyptra orci, Ehrenberg, 1872, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 293, Taf. x. figs. 9, 10.

Shell flatly campanulate, very similar to the preceding species in general form and fenestration, but flatter and smaller, with different proportions. Length of the three joints = 7 : 2 : 1, breadth = 1 : 6 : 8. Cephalis hemispherical, very small, with two divergent, bristle-shaped horns of the same length. Abdomen nearly horizontally expanded.

Dimensions.—Length of the three joints, a 0.015, b 0.04, c 0.01; breadth, a 0.02, b 0.12, c 0.16.

Habitat.—Indian Ocean, Zanzibar, depth 2200 fathoms (Pullen).

3. Theocalyptra discoides, Haeckel.

Cycladophora discoides, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 68, Taf. xviii. fig. 4.

Shell flatly campanulate, similar to the two preceding species, but not so delicate, with thicker bars, and roundish, not hexagonal meshes. Length of the three joints = 1 : 2 : 2, breadth = 3 : 6 : 9.