Definition.—Coronida with eight large, partly fenestrated gates of equal size. Skeleton composed of three complete latticed rings, perpendicular to one another.
The genus Trissocyclus differs from its nearly allied ancestral form Trissocircus only in the development of small anastomosing branches along the edges of the rings. The incomplete lattice-work, produced by these anastomoses, partly protects the eight large gates. The three rings are either of equal or of different sizes.
Subgenus 1. Tricyclarium, Haeckel.
Definition.—Sagittal ring circular, smaller than the other two rings, which are both elliptical.
1. Trissocyclus stauroporus, n. sp. (Pl. [83], fig. 5).
Sagittal ring circular, smaller than the other two equal elliptical rings. On each of the six corners (or crossing points of every two rings) are developed four small pores, forming a cross around the poles of the three axes. All three rings smooth, without spines.
Dimensions.—Height of the frontal ring 0.09, breadth 0.13.
Habitat.—Central Pacific, Station 265, depth 2900 fathoms.
2. Trissocyclus triaxonius, n. sp.
Sagittal ring circular, smaller than the two other equal elliptical rings. From the edges of all three rings are developed small branches, which by irregular ramification and anastomoses form small irregular meshes along the rings. Similar to Trissocircus octostoma, but with thicker rings; it may be developed from the latter species by connection of the branches which border the rings.