Page:Scientific results HMS Challenger vol 18 part 2.djvu/384

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Definition.—Pyramidal shell with numerous (twelve or more) radial main beams (commonly twelve to eighteen, sometimes twenty to thirty or more).

11. Plectopyramis polypleura, n. sp. (Pl. 56, fig. 8).

Shell smooth, slenderly conical, with straight outlines. Cephalis small, subspherical, with very small dot-like pores, and an oblique conical horn twice the length. Thorax with numerous (twenty to thirty) divergent longitudinal ribs; usually twelve of these radial beams appear as stronger, primary main ribs, and twelve to eighteen other as secondary, feebler, interpolated ribs. They are crossed by numerous (twenty to forty or more) horizontal rings. The large meshes so produced are quadrangular, and partly filled up by a very delicate secondary network, with small square pores.

Dimensions.—Cephalis 0.03 diameter; thorax 0.36 long, 0.24 broad.

Habitat.—Central Pacific, Station 267, depth 2700 fathoms.

12. Plectopyramis lagena, n. sp.

Shell smooth, wine-bottle shaped, or slenderly conical-campanulate, with bent outlines, which are concave in the upper, convex in the lower half. Cephalis very small, spherical, hyaline, without pores. Thorax with twenty-four radial ribs, twelve stronger primary, alternating with twelve feebler secondary. They are crossed by interrupted transverse bars. The irregular quadrangular meshes so produced are filled up by a delicate, irregular, secondary network.

Dimensions.—Cephalis 0.02 diameter; thorax 0.5 long, 0.3 broad.

Habitat.—South Atlantic, Station 335, depth 1425 fathoms.

Definition.— (vel Dicyrtida multiradiata aperta) with numerous straight or slightly curved radial ribs in the wall of the pyramidal thorax. Network spongy. Cephalis commonly without horn.

The genus Spongopyramis has arisen from Sethopyramis by development of an irregular spongy framework around the lattice-shell, and is one of the small number of in which the shell-wall exhibits a spongy structure. In both observed species the cephalis is small, but evident.