Page:Scientific results HMS Challenger vol 18 part 2.djvu/39

Rh opposed branches (the ascending branch of the basal rod and the descending branch of the apical spine), which in the preceding species nearly compose a sagittal ring, are here absent.

Dimensions.—Length of the two larger 0.27, of the two smaller 0.21.

Habitat.—North Atlantic, Iceland (Steenstrup), surface.

Definition.— with six radial spines.

Definition.— with six radial spines, arising from one common central point.

The genus Hexaplagia differs from the preceding genera of Plagonida in the possession of six radial spines, diverging from one common central point. Commonly, these six spines seem to lie opposite in pairs in three different meridian planes, and in this case Hexaplagia may be regarded as a Plagiacantha, the three radial spines of which are prolonged over the basal pole of the central capsule. But in other species the six spines seem to lie in different planes. Further observations are required.

1. Hexaplagia arctica, n. sp.

Spines opposite in pairs in three diameters, crossed in the common centre, straight, three-sided prismatic, with dentate edges. The three pairs are equal, but the upper spine of each pair only half as long as the lower spine.

Dimensions.—Length of the upper spine 0.13, of the lower 0.27.

Habitat.—Arctic Ocean (Greenland), in the stomach of a Medusa (Olrik).

2. Hexaplagia antarctica, n. sp.

Spines opposite in pairs in three diameters, crossed in the common centre, slightly curved, three-sided prismatic, with verticillate branches; the three pairs are equal, but the lower spine of each pair bears three to four verticils, each of three branches, and is two to three times as long as the upper spine, which bears two verticils only.

Dimensions.—Length of the upper spine 0.12, of the lower 0.3.

Habitat.—Antarctic Ocean, Station 157, depth 1950 fathoms.