The genus Plectaniscus has probably been derived from Plagoniscus by reticular union of the neighbouring branches of the four radial spines, and exhibits therefore to it the same relation that Tetraplecta bears to Tetraplagia. Whilst in these two latter genera the four spines are equal, in the two former genera there is an important difference between a vertical spine (or apical horn) and three divergent (commonly larger) spines, corresponding to the three basal feet of the triradiate Nassellaria. Perhaps, therefore, Plectaniscus is one of the remotest ancestors of the latter; but differs in the absence of a complete lattice-shell.
1. Plectaniscus cortiniscus, n. sp. (Pl. [91], fig. 9).
Spines straight, three-sided prismatic, with three to four verticils of short perpendicular branches; the branches of the distal verticils are simple and free, those of the proximal verticils again ramified and connected by an arachnoidal network of delicate threads. The vertical apical spine (or horn) is scarcely one-third or half as long as the three divergent basal spines (or feet), and the angle between it and the latter is smaller.
Dimensions.—Length of the apical spine 0.05 to 0.1, of the three basal spines 0.18 to 0.2.
Habitat.—North Pacific, Station 244, surface.
2. Plectaniscus tripodiscus, n. sp.
Spines curved, cylindrical, with a variable number of irregular branches, which in the distal half of the spines are free, in the basal half again ramified and connected by a loose spongy framework. The apical spine is shorter and simpler, nearly straight, less ramified.
Dimensions.—Length of the apical spine 0.1, of the three basal spines 0.15 to 0.18.
Habitat.—North Pacific, Station 238, surface.
3. Plectaniscus archiscenium, n. sp.