Page:Scientific results HMS Challenger vol 18 part 1.djvu/963

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Definition.— with two branched, but not latticed, opposite apophyses on each radial spine.

The genus Phractacantha differs from its ancestral form, the preceding Lithophyllium by the ramification of the apophyses, which are either forked or bear lateral branches. If the prolonged fork-branches of the neighbouring spines meet and form a lattice-shell, this genus passes over into Phractaspis, the common ancestral form of the Diporaspida.

1. Phractacantha bifurca, n. sp.

Spines cylindrical, thin, of equal breadth throughout their whole length. Apex simple. Base pyramidal, without leaf-cross. From the outer third arise two opposite, thin, forked apophyses; ends of the fork branches acute.

Dimensions.—Length of the spines 0.1, breadth 0.006.

Habitat.—Central Pacific, Station 274, surface.

2. Phractacantha bipennis, n. sp.

Spines compressed, two-edged, tapering from the broader middle towards the two ends. Apex simple. Base pyramidal, without leaf-cross. From the middle arise two opposite broad, forked apophyses; ends of the fork-branches broad and obtuse.

Dimensions.—Length of the spines 0.15, breadth in the middle 0.012.

Habitat.—Central Pacific, Station 266, surface.

Definition.— with two latticed or fenestrated apophyses on each radial spine.

The genus Doracantha arises from the foregoing Phractacantha by union of the fork-branches of the apophyses on each spine. By this concrescence is formed a polygonal or roundish plate with two pores, pierced by the radial spine between them. Doracantha may be regarded as a Dorataspis, in which the twenty small fenestrated plates have not become united.