The genus Spongothamnus differs from the preceding Spongechinus in the ramification of the numerous radial spines, covering the surface of the hollow spongy sphere; it therefore exhibits the same relation to the latter that Spongodrymus bears to Spongiomma.
1. Spongothamnus furcatus, n. sp.
Spongy sphere twice as broad as its inner cavity, with very delicate bars, and irregular, dense framework. From the surface arise one hundred and fifty to one hundred and eighty thin, forked, bristle-shaped spines, half as long as the radius; both fork branches one-third as long as the basal or simple part.
Dimensions.—Diameter of the sphere 0.3, of its inner cavity 0.15; length of the spines 0.08.
Habitat.—North Pacific, Station 241, surface.
2. Spongothamnus scoparius, n. sp.
Spongy sphere four times as broad as its inner cavity, with thick bars and rather loose framework. From the surface arise sixty to eighty broom-shaped radial spines, as long as the radius, each in the basal half simple, in the distal half with six to twelve irregularly ramified branches (similar to the spines of Cromyodrymus abietinus, Pl. [30], fig. 6).
Dimensions.—Diameter of the sphere 0.4, of the inner cavity 0.1; length of the spines 0.2.
Habitat.—Central Pacific, Station 271, surface.