The genus Toxarium differs from its ancestral form Acrocubus in the development of two pairs of peculiar bows, placed in the lateral or frontal plane; the upper pair projects above the mitral gates, and may be called the "galear bows," the lower pair projects below the basal gates of the cubical shell, and may be called the "thoracal bows." Both pairs are produced originally by the development of two forked vertical spines, arising in the frontal plane on both poles of the principal axis from the sagittal ring (compare Protympanium amphipodium, Pl. [93], fig. 14). The apical fork-branches become connected with the lateral corners of the mitral gates, the basal branches with those of the basal gates. By development of lattice-work between the bows and the rings some Spyroidea (Amphispyris, &c.) may have arisen. The important genus Toxarium may be divided into three subgenera. In Toxellium all the bows are simple, in Toxonium, all are forked; in Toxidium, the galear bows are simple, the thoracal bows forked.
Subgenus 1. Toxellium, Haeckel.
Definition.—Galear and thoracal bows simple.
1. Toxarium circospyris, n. sp. (Pl. [88], fig. 1).
Galear and thoracal bows simple, of about the same size as the frontal bows between them; all bows armed with scattered simple conical spines. Sagittal constriction deep, half as long as the whole shell. Lateral outlines of the shell nearly parallel.
Dimensions.—Length of the shell 0.16, breadth 0.13.
Habitat.—Central Pacific, Stations 265 to 274, depth 2350 to 2925 fathoms.
2. Toxarium subcirculare, n. sp.
Galear and thoracal bows simple, armed with scattered simple spines of about equal size, smaller than the smooth frontal bows between them. Sagittal constriction very flat. Therefore the frontal perimeter of the shell is nearly circular.
Dimensions.—Length of the shell 0.15, breadth 0.14.