therefore be regarded as the common ancestral form of this family. The simple skeleton is composed of two meridional rings only, perpendicular one to another—a sagittal and a frontal ring.
Subgenus 1. Zygostephus, Haeckel, 1881, Prodromus, p. 446.
Definition.—The two vertical rings of nearly equal height, therefore without sagittal constriction.
1. Zygostephanus dissocircus, n. sp. (Pl. 93, fig. 1).
Both rings elliptical, of nearly equal height, smooth, without spines. Frontal ring somewhat longer than the sagittal. No sagittal constriction.
Dimensions.—Height of the rings 0.07, breadth 0.1.
Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific, surface.
2. Zygostephanus serratus, n. sp.
Both rings elliptical, of nearly equal height, with a serrate prominent edge on the outer convexity (in the transverse section three-edged). No sagittal constriction.
Dimensions.—Height of the rings 0.08, breadth 0.12.
Habitat.—Central Pacific, Station 266, depth 2750 fathoms.
3. Zygostephanus mülleri, Haeckel.
Both rings elliptical, of nearly equal height, armed with numerous (about fifty) slender curved spines, as long as the radius of the rings; commonly each ring with twelve pairs of divergent spines. No sagittal constriction.
Dimensions.—Height of the rings 0.06, breadth 0.1.
Habitat.—Mediterranean (Messina), surface.
4. Zygostephanus ramosus, n. sp.
Both rings elliptical, of nearly equal height, armed with numerous branched spines, which are arranged on each ring in two divergent rows; the branches of the spines are curved and partly protect the open gates. No sagittal constriction.
Dimensions.—Height of the rings 0.07, breadth 0.09.
Habitat.—North Pacific, Station 244, depth 2900 fathoms.
