Page:Scientific results HMS Challenger vol 18 part 1.djvu/748

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Definition.— with four simple, undivided, chambered arms, without a patagium; quadrangular shell a regular cross, with four equal arms placed at right angles.

The genus Stauralastrum is the most simple form of the Tessarastrida, or of those Porodiscida in which the margin of the central disk is armed with four chambered arms. In Stauralastrum these four arms are quite simple and equal, without a patagium, separated by four right angles, so that the whole shell represents a regular rectangular cross. If we connect the distal points of the arms by lines, we get a complete square. (In my Prodromus, 1881, the species of this genus were united with Hagiastrum, which genus I now retain for the simple bilateral Tessarastrida.)

Definition.—Ends of the arms blunt, without terminal spines.

1. Stauralastrum cruciforme, n. sp. (Pl. 45, fig. 6).

Arms very thin, nearly linear, four to five times as long as broad, of equal breadth at the base and at the truncated distal end; their breadth equals one-third of the radius of the central disk. Edges of the arms parallel.

Dimensions.—Radius of each arm 0.8, breadth 0.016.

Habitat.—South Pacific, Station 293, surface.

2. Stauralastrum lanceolatum, n. sp.

Arms lanceolate, three times as long as broad, in their middle part three times as broad as at both ends; their greatest breadth nearly equals the diameter of the central disk. (The arms have the same form as in Euchitonia lanceolata, Pl. 43, fig. 9.) Edges of the arms convex.

Dimensions.—Radius of each arm 0.3, greatest breadth (in the middle part) 0.08.

Habitat.—Pacific, central area, Station 273, depth 2350 fathoms.

3. Stauralastrum ordo, n. sp.

Arms trapezoid, about as long as broad, twice as broad at their truncated distal end as at the base; their basal breadth equals the radius of the central disk, which exhibits two to three rings. (The arms have nearly the same form as those in Hagiastrum mosis, Pl. 45, fig. 3.) Edges of the arms rectilinear, divergent towards the ends.

Dimensions.—Radius of each arm 0.12, basal breadth 0.04, distal breadth 0.08.

Habitat.—Pacific, central area, Station 265, depth 2900 fathoms.