Page:Scientific results HMS Challenger vol 18 part 2.djvu/270

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Definition.— with the basal mouth of the shell fenestrated (vel Monocyrtida triradiata clausa).

Definition.— (vel Monocyrtida triradiata clausa) with a free simple internal columella, prolonged into an apical horn. Three basal feet free, without lateral wings.

The genus Euscenium and the three following nearly allied genera form together the remarkable small group of Euscenida, probably one of the oldest and most primitive of all, and perhaps the common root of this suborder, arising directly from the. The simple skeleton is composed of four radial beams united in one point (the cortinar centrum). Three beams are directed downwards, and form three divergent basal feet; the fourth beam is directed upwards and represents an apical horn; its basal part is enclosed as "columella" in the cavity of the simple shell, the lattice-work of which connects the four spines (compare above, Plectaniscus, p. 924). In the genus Euscenium, the simplest of all, the columella is simple, and the three feet also simple, without wings.

Definition.—Three feet simple, not branched.

1. Euscenium plectaniscus (Pl. 98, fig. 1).

Shell three-sided pyramidal, thorny, about as long as broad, with three dentate edges. Framework very loose, with irregular polygonal meshes; in the middle of each of the three triangular sides a horizontal band of denser network. Basal plate with three very large simple collar-holes. The apical horn and the three feet are three-sided pyramidal, about half as long as the columella, with three prominent irregularly dentate edges. The horn is straight, whilst the three feet are hook-shaped, its distal end being curved downwards and inwards.

Dimensions.—Shell 0.11 long, 0.13 broad.

Habitat.—Central Pacific, Station 274, depth 2750 fathoms.

2. Euscenium archicyrtis, n. sp.

Shell three-sided pyramidal, smooth, nearly as long as broad, with three straight edges connected by irregular loose framework with polygonal meshes. Basal plate with three very large