Page:Scientific results HMS Challenger vol 18 part 2.djvu/103

Rh

Definition.— with six large, partly latticed gates (four lateral and two basal). Skeleton composed of two complete rings (the sagittal and basal) and one incomplete (frontal) ring. Basal ring without developed typical feet.

The genus Plectocoronis differs from the preceding Eucoronis, its ancestral form, in the development of loose irregular lattice-work along the two meridional rings, and therefore bears to it the same relation that Acanthodesmia does to Coronidium. If the lattice-work become more complete and close the gates, these Coronida pass over directly into (Zygospyrida).

1. Plectocoronis anacantha, n. sp.

Basal ring and frontal ring elliptical, without sagittal constriction. Sagittal ring ovate. The frontal and sagittal rings are bordered on both edges with an incomplete series of small irregular pores. All three rings smooth, without spines.

Dimensions.—Height of the frontal ring 0.1, breadth 0.16.

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

2. Plectocoronis triacantha, n. sp.

Basal ring and frontal ring nearly circular, without sagittal constriction. Sagittal ring elliptical, prolonged at the apex into an apical horn from which two branched lateral bows descend and unite with the uppermost parts of the frontal ring. Four other small bows descend from the lowermost parts of the latter and unite with the basal ring. From the lateral corners of the basal ring descend two short vertical feet. (May be regarded as derived from Podocoronis toxarium, Pl. 83, Fig. 7, by development of the bows connecting the rings.)

Dimensions.—Height of the frontal ring 0.11, breadth 0.13.

Habitat.—Central Pacific, Station 265, depth 2900 fathoms.

3. Plectocoronis pentacantha, n. sp. (Pl. 93, fig. 6).

Basal ring and frontal ring kidney-shaped, with deep sagittal constriction. Sagittal ring ovate, prolonged at the apex into an oblique horn. From the edges of the angular rings delicate threads arise, which anastomose and form irregular arachnoidal meshes, partly closing the four open lateral gates. Two basal gates simple. From the four nodal points of the basal ring there arise four, short diverging feet, comparable to rudimentary cortinar feet (two lateral from the frontal ring and two sagittal from the primary ring, as basal prolongations).

Dimensions.—Height of the frontal ring 0.12, breadth 0.18.

Habitat.—Central Pacific, Station 271, surface.