The genus _Trissocircus_, and the following nearly allied _Trissocyclus_, differ from the two preceding ancestral genera in the remarkable growth of the four basal gates, which reach the size of the lateral gates. Therefore both hemispheres of the shell (the upper or apical and the lower or basal) here become equal and the basal ring becomes equatorial. Sometimes even all three rings attain the same size, so that it is difficult or impossible to distinguish them. In this curious case the Coronida exhibit a striking resemblance to some #Sphæroidea#. Subgenus 1. _Tricircarium_, Haeckel. _Definition._--Sagittal ring smaller than the two other rings, which are both elliptical. 1. _Trissocircus lentellipsis_, n. sp. (Pl. 93, fig. 10). All three rings elliptical, of different sizes, smooth, without spines. The sagittal ring is the smallest, but two to three times as thick as the other two rings, which have the larger (transverse) axis common. The smaller (sagittal) axis of the sagittal ring is also the smaller axis of the equatorial ring, whilst the larger (principal) axis of the former is the smaller axis of the frontal ring. _Dimensions._--Height of the frontal ring 0.07, breadth 0.1. _Habitat._--Western Tropical Pacific, Station 224, depth 1850 fathoms. 2. _Trissocircus binellipsis_, n. sp. (Pl. 83, fig. 6). Sagittal ring circular, smaller than the other two rings, which are both equal, elliptical, slightly constricted on the poles of the principal and transverse axes. The axis of the circular sagittal {986}ring is the shorter axis of the frontal and equatorial ring, and about half as long as their longer axis. All three rings smooth, without spines. _Dimensions._--Height of the frontal ring 0.08, breadth 0.12. _Habitat._--Central Pacific, Station 268, depth 2900 fathoms. 3. _Trissocircus octostoma_ (Pl. 93, fig. 11). Sagittal ring circular, smaller than the other two rings, which are both equal, elliptical, distinctly constricted at the poles of the principal and transverse axes. The axis of the circular sagittal ring is the shorter axis of the frontal and equatorial ring, about one-third shorter than their longer axis. All three rings armed with short, irregularly branched spines. _Dimensions._--Height of the frontal ring 0.1, breadth 0.12. _Habitat._--Indian Ocean, Ceylon (Belligemma), Haeckel, surface. Subgenus 2. _Tricirconium_, Haeckel. _Definition._--All three rings equal, circular. 4. _Trissocircus octahedrus_, n. sp. All three rings equal, circular, smooth, their rods cylindrical (in the transverse section circular). From each pole of the three equal axes arises a short conical spine; these six spines correspond to the six corners of a regular octahedron. _Dimensions._--Diameter of all three rings 0.08, thickness 0.006. _Habitat._--Central Pacific, Station 271, depth 2425 fathoms. 5. _Trissocircus globus_, n. sp. (Pl. 82, fig. 13).