_Dimensions._--Height of the frontal ring 0.07 to 0.09, breadth 0.17 to 0.2. _Habitat._--Fossil in Barbados. Subgenus 2. _Tristephaniscus_, Haeckel, 1881, Prodromus, p. 445. _Definition._--Sagittal and frontal ring of equal size and form. 4. _Tristephanium quadricorne_, n. sp. (Pl. 93, fig. 7). Sagittal and frontal ring of equal size and form, larger than the circular basal ring. From the four corners, in which the latter crosses the two former, arise four strong, divergent spines, branched like a deer's antler. Some smaller spines are scattered on the rings, and a bunch of four spines arises on the apical pole. The four upper gates are triangular, the four lower nearly semicircular, the latter of equal size, half as large as the former. _Dimensions._--Height of the frontal ring 0.13, breadth 0.14. _Habitat._--South Atlantic, Station 342, depth 1445 fathoms. Genus 422. _Tricyclidium_,[41] Haeckel, 1881, Prodromus, p. 446. _Definition._--#Coronida# with eight large, partly fenestrated gates; the four upper or lateral gates larger than the four lower or basal gates. Skeleton composed of three latticed complete rings, perpendicular to one another. The genus _Tricyclidium_ differs from the preceding _Tristephanium_, its ancestral form, in the development of loose rudimentary lattice-work along the rings, and therefore bears to it the same relation that _Plectocoronis_ does to _Eucoronis_. It may pass directly over into _Dictyospyris_. 1. _Tricyclidium dictyospyris_, n. sp. (Pl. 93, fig. 13). Sagittal ring ovate, twice as thick as the two other larger rings, which become very thin, thread-like at the lateral junction, and are both slightly violin-shaped, in the middle sagittal plane constricted; the frontal ring larger than the basal. All three rings bear small scattered spines, which are irregularly branched, and by anastomoses of the thread-like branches form small irregular meshes along the rings. The four basal gates are of equal size. _Dimensions._--Height of the frontal ring 0.11, breadth 0.15. _Habitat._--Equatorial Atlantic, Station 347, depth 2250 fathoms. {985}2. _Tricyclidium semantrum_, n. sp. Sagittal ring ovate, smaller than the violin-shaped frontal ring and larger than the elliptical basal ring. All three rings of nearly equal thickness, armed with short irregular branches, which are partly connected, and forming small irregular meshes along the rings. The four basal gates are of different sizes; the two anterior (jugular) gates only half as large as the two posterior (cardinal) gates. _Dimensions._--Height of the frontal ring 0.12, breadth 0.18. _Habitat._--South Atlantic, Station 348, depth 2450 fathoms. Genus 423. _Trissocircus_,[42] Haeckel, 1881, Prodromus, p. 446. _Definition._--#Coronida# with eight large simple gates of equal size. Skeleton composed of three simple complete rings, perpendicular to one another.