3. Spongodiscus resurgens, Ehrenberg.
Spongy disk lenticular, biconvex, in the darker centre much thicker than towards the thin periphery, with an irregular framework, without concentric rings, but with numerous piercing radial beams. Texture equal; meshes three to four times as broad as the bars.
Dimensions.—Diameter of the disk 0.1 to 0.3, of the meshes 0.003 to 0.006.
Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific, surface; also fossil in the Tertiary rocks of Sicily and Barbados.
4. Spongodiscus favus, Ehrenberg.
Spongy disk lenticular, biconvex, in the darker centre much thicker than towards the periphery, with an irregular framework, without concentric rings. Texture different, in the outer half with numerous radial beams and loose network, the meshes of which are three to four times as large as in the darker and denser framework of the centre.
Dimensions.—Diameter of the disk 0.2, of the outer meshes 0.008 to 0.012, of the inner meshes 0.002 to 0.003.
Habitat.—North Atlantic, Greenland, Færöe Channel (John Murray), surface.
5. Spongodiscus biconcavus, n. sp.
Spongy disk biconcave, in the peripheral, ring-like, thickened part twice as thick as in the hollowed central part, with an irregular framework, without concentric rings. Texture different, in the outer half looser than in the inner, darker part; meshes of the outer part five to six times, of the inner twice to three times, as broad as the bars.
Dimensions.—Diameter of the disk 0.25, of the outer meshes 0.01 to 0.012, of the inner 0.004 to 0.006.
Habitat.—Pacific, central area, Station 265, depth 2900 fathoms.
Subgenus 2. Spongocyclia, Haeckel, 1862, Monogr. d. Radiol., p. 469.
Definition.—Spongy framework of the disk in the inner part with concentric, circular rings, in the outer part irregular.
