Subgenus 2. Histiastromma, Haeckel.
Definition.—Distal ends of the arms spiny, furnished with one or more terminal spines.
4. Histiastrum quaternarium, Ehrenberg.
Histiastrum quaternarium, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 74, Taf. xxiv. figs. 3, 4.
Arms six times as long as broad at their base, with rectilinear, little divergent edges; at their truncated distal end a little broader, with one single, very strong, conical, terminal spine. Central disk with four to five rings, somewhat broader than the arm. Patagium incomplete, enveloping only the basal half of the arms.
Dimensions.—Radius of each arm 0.18, basal breadth 0.03, distal breadth 0.05.
Habitat.—Fossil in the rocks of Barbados.
5. Histiastrum gladiatum, n. sp.
Astromma sp., Bury, 1862, Polycystins of Barbados, pl. v. fig. 1.
Arms triangular, eight times as long as broad at their base; at their distal end three times as broad as at their base, with rectilinear, divergent edges; their truncated end with a large conical, terminal spine. Central disk with four to five rings, broader than the arms. Patagium incomplete, enveloping only the basal half of the arms.