Page:Scientific results HMS Challenger vol 18 part 1.djvu/755

Rh central chamber. Patagium complete, spongy, with radiating beams, enveloping the whole disk, with the exception of the outermost end of the arms, which is armed with twelve to sixteen strong conical spines, the middle (perradial) spine much larger.

Dimensions.—Radius of each arm (without terminal spine) 0.18, basal breadth 0.025, terminal breadth 0.08.

Habitat.—South Atlantic, Station 333, surface.

Definition.— with four simple, undivided, chambered arms, connected by a patagium; quadrangular shell bilateral, two opposite arms of the main axis (or principal arms) different from the two others (or lateral arms).

The genus Tessarastrum, formerly united by me with Histiastrum, differs from the latter in its bilateral or symmetrical form, and bears therefore the same relation to it that Hagiastrum does to Stauralastrum.

Definition.—Both longitudinal arms of equal size and form.

1. Tessarastrum straussii, n. sp. (Pl. 45, fig. 8).

Histiastrum straussii, Haeckel, 1881, Prodromus et Atlas (pl. xlv. fig. 8).

Cross not rectangular. Both principal arms of equal size and form, four times as long as broad, and twice as long as the broader lateral arms; the former with ten to eleven, the latter with five to six joints, separated by convex transverse septa. Distal ends of the arms blunt. Axes of the smaller arms not perpendicular to that of the larger arms; therefore the anterior angles between them smaller than the posterior angles. Patagium between the arms incomplete. I call this remarkable species after the great German philosopher David Strauss.

Dimensions.—Radius of the principal arms 0.24, of the lateral arms 0.12; greatest breadth (in the middle) of the former 0.05, of the latter 0.06.

Habitat.—North Pacific, Hyalonema-ground, March 5, 1875.

2. Tessarastrum spinozæ, n. sp.

Cross rectangular. Both principal arms of equal size and form, ten times as long as broad, and twice as long as the lateral arms, which are only five times as long as broad. All arms linear, at their distal end club-shaped, and armed with twenty to thirty very strong angular spines. Patagium incomplete, enveloping only the basal half of the arms. (Resembles Histiastrum boseanum,