The genus Semantrum, one of the most important of the Nassellaria, arises from the preceding Semantis by duplication of the two basal gates. Behind the pair of coracal rods there arises from the basilar rod of the sagittal ring a third pair of lateral horizontal branches, the scapular rods. These become connected with the coracal rods on each side, and so produce a second posterior pair of basal pores, the "cardinal gates." These are constantly larger than the anterior "jugular gates." Therefore the vertical ring of Semantrum possesses a horizontal basal ring with four very characteristic gates, enclosed by three pairs of lateral curved and connected branches, and these become transmitted by heredity to the majority of the Nassellaria.
1. Semantrum quadrifore, n. sp. (Pl. [92], fig. 5).
Sagittal ring subcircular or ovate, with three edges and four sagittal forked spines on the odd edge (two dorsal and two ventral spines). Basal ring decagonal, with ten simple or forked spines on the ten corners. Jugular gates tetragonal. Cardinal gates pentagonal.
Dimensions.—Height of the sagittal ring 0.09, breadth 0.07.
Habitat.—Tropical Atlantic, Station 351, surface.
2. Semantrum tetrastoma, n. sp. (Pl. [92], fig. 3).
Sagittal ring semicircular, thorny, without edges. Basal ring tetragonal or nearly cordate, with a small anterior and a large posterior bow-shaped incision; on the lateral edges with numerous irregular thorns. Jugular gates pear-shaped. Cardinal gates nearly kidney-shaped.
Dimensions.—Height of the sagittal ring 0.1 to 0.14, breadth 0.07 to 0.09.
Habitat.—Central Pacific, Stations 265 to 268, depth 2700 to 2900 fathoms.
3. Semantrum tetrapylum, n. sp.