_Definition._--#Coronida# with four large, simple, lateral gates, without basal gate and lattice-work. Skeleton composed of two simple meridional rings, perpendicular to one another. The genus _Zygostephanus_, founded by me in 1862 for the Mediterranean _Zygostephanus mülleri_, is the simplest and most primitive of all the Coronida, and may {971}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. _Zygostephanus mülleri_, Haeckel, 1862, Monogr. d. Radiol., p. 268, Taf. xii. fig. 2. 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. {972}Subgenus 2. _Zygostephaniscus_, Haeckel, 1881, Prodromus, p. 446. _Definition._--The two vertical rings of different heights; the frontal ring higher than the sagittal ring, therefore with a sagittal constriction in the median plane. 5. _Zygostephanus reniformis_, n. sp. Frontal ring kidney-shaped, in the upper half convex, in the lower concave, in the latter with a deep sagittal constriction, armed with numerous irregular short spines. Sagittal ring thicker and lower, only two-thirds as high, with four pairs of short divergent spines (two ventral and two dorsal pairs). _Dimensions._--Height of the frontal ring 0.13, breadth 0.2. _Habitat._--Central Pacific, Station 271, depth 2425 fathoms. 6. _Zygostephanus violina_, n. sp.