Sagittal ring obliquely ovate, thorny. Basal ring kidney-shaped or nearly pentagonal, with broad concave dorsal incision and five larger branched spines on the five corners; between them smaller thorns. Jugular gates pear-shaped, scarcely half as broad as the triangular cardinal gates. _Dimensions._--Height of the sagittal ring 0.12, breadth 0.09. _Habitat._--Central Pacific, Station 271, depth 2425 fathoms. Genus 409. _Semantidium_,[28] n. gen. _Definition._--#Semantida# with six basal pores (two anterior jugular, two middle cardinal, and two posterior cervical pores), without typical basal feet. The genus _Semantidium_ exhibits a further developmental stage of the basal plate of the Semantida. Whilst this plate in _Semantis_ possesses one pair of basal pores, and in _Semantrum_ two pairs, here in _Semantidium_ it has three pairs. The new third pair is produced on the dorsal edge of the basal plate, which becomes connected with the basal part of the dorsal rod of the sagittal ring by a pair of cervical rods. The middle pair of pores (the cardinal) are always larger than the anterior (jugular) and the posterior (cervical pores). The same form of basal plate is preserved in numerous #Spyroidea# and #Cyrtoidea#, as a "cortinar septum with six collar pores," (_e.g._, Pl. 53, fig. 18). 1. _Semantidium hexastoma_, n. sp. (Pl. 92, fig. 6). Sagittal ring ovate, thorny. Basal ring rhombic or nearly square, with four short conical descending spines on the four prominent edges (two sagittal and two lateral); between them numerous smaller irregular thorns. Jugular and cervical gates nearly equal, ovate, half as broad as the triangular cardinal gates between them. _Dimensions._--Height of the sagittal ring 0.14, breadth 0.1. _Habitat._--Central Pacific, Station 265, depth 2900 fathoms. 2. _Semantidium sexangulum_, n. sp. Sagittal ring ovate, with six pairs of short branched thorns (two apical, two dorsal, and two ventral). Basal ring hexagonal, with three pairs of short branched thorns on the six corners (one {961}sagittal pair, one posterior and one anterior). All six gates of the basal plate triangular, the jugular and cervical a little smaller than the cardinal gates. _Dimensions._--Height of the sagittal ring 0.1, breadth 0.07. _Habitat._--Tropical Atlantic, Station 348, depth 2450 fathoms. 3. _Semantidium haeckelii_, Bütschli. Stephanolithis Haeckelii, Bütschli, 1882, Zeitschr. f. wiss. Zool., vol. xxxvi. pp. 499, 538, Taf. xxxii. figs. 6_a_, 6_b_. Sagittal ring elliptical, with three pairs of short horizontal branched spines, one apical and two equatorial pairs (one dorsal and one ventral). Basal ring roundish hexagonal, with numerous short thorns on the margin. Jugular pores ovate, about half as broad as the ovate cardinal pores and twice as broad as the small cervical pores. _Dimensions._--Height of the sagittal ring 0.08, breadth 0.06. _Habitat._--Fossil in Barbados.