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 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.