Page:Scientific results HMS Challenger vol 18 part 2.djvu/882

1758 2. Cœloplegma tritonis, n. sp. (Pl. 127, figs. 2-13).

Shell-mantle one and a quarter times as long as broad, its frontal perimeter heptagonal, with seven concave sides (fig. 2), its sagittal perimeter ovate, its zonal perimeter nearly square (fig. 3). Odd nasal style with twelve to fourteen, the paired pectoral styles with three to four, lateral styles with six to eight, and tergal styles with eight to ten pairs of branches. Coronets three times forked, each with eight terminal branches, bearing a small spinulate knob. This remarkable species, in external appearance very different from the preceding, is connected immediately with it by numerous transitional varieties; both species represent the opposite terminal poles of a long series of "Darwinian metamorphic forms." If only the two specimens, figured in Pl. 127, figs. 1 and 2, were known, every one would distinguish them as two widely different species. But the careful comparison of numerous intermediate forms demonstrates that there is no "missing link" in this long and remarkable chain. The careful comparative study of these very variable and most highly developed Cœlographida may be regarded as a strong argument for the theory of descent, and explains the true "origin of species."

Dimensions.—Length of the shell 1.8 to 2.5, breadth 1.5 to 2.1.

Habitat.—North Atlantic, Gulf Stream, off the Færöe Channel, in depths between 40 and 200 fathoms, John Murray.

3. Cœloplegma tetradecastylum, n. sp.

Shell-mantle one and a third times as long as broad, its frontal perimeter heptagonal, with seven concave sides. Similar to the preceding species, differs from it mainly in the prolonged nasal style, which bears sixteen to eighteen pairs of branches, and is about three times as long as each of the six paired styles, each of which bears five to six pairs of branches. Coronets three times forked, each with eight terminal branches, armed with scattered spines, and bearing a knob with three recurved hooks.

Dimensions.—Length of the shell 3.2, breadth 2.4.

Habitat.—Tropical Atlantic, Stations 335 to 342, surface.

4. Cœloplegma atlanticum, n. sp.

Shell-mantle one and a half times as long as broad, its frontal perimeter ovate. Similar to Cœloplegma murrayanum (Pl. 127, fig. 1), differs from it in the different length of the styles; the nasal odd style (with eighteen to twenty pairs of branches) is about twice as long as the lateral styles (with ten to twelve pairs), and three times as long as the pectoral and tergal styles (with six to eight pairs). Coronets four times forked, each with sixteen terminal branches, armed with recurved spines, and bearing a cross of four curved hooks.

Dimensions.—Length of the shell 2.5, breadth 1.7.

Habitat.—South Atlantic, Station 318, depth 2040 fathoms.